Blood magic is hardly a new concept to the fantasy genre, but how it is handled in Earthdawn is a unique approach to the subject. Typically, blood magic is portrayed as dangerous and negative, something that villains do and heroes may turn to as an option of last resort (most commonly as an aspect of martyrdom - a noble sacrifice, lesser of two evils, or ends justify the means). This is generally even the case in the parent setting, Shadowrun. Blood magic is used almost exclusively by Aztechnology, a corp that may mildly be described as "villainous".
In Earthdawn, Magic is a part of everyday life and blood magic is simply another kind of magic; not a wholly uncommon one at that. Everything must be powered somehow: common magic uses alchemy, true elements and other discrete things, though they tend to not be very powerful; spells and thread magic derive their power from astral space, though this only available to those who can work this kind of magic (adepts); and blood magic is powered by your life essence. Every adept will engage in blood magic through Strain, perhaps frequently, while even non-adepts may make Blood Oaths or use Blood Charms. Shadowrun even has hints of these elements, the Great Ghost Dance has been indicated to be a ritual powered by blood magic, and the Dragonheart. Drain from spells could be argued to be an example of blood magic, even if it isn't recognized as such (the magic level in the 6th World is considerably lower, thus spells require a great deal more effort).
There are a few kinds of blood magic that characters in Earthdawn will likely encounter: Blood Charms, Blood Oaths and Sacrifice Magic.
This damage is the primary balancing factor when considering how many Blood Charms you want to load up on. Combat characters can handle more and are generally in need of more; they also tend to be more geared at that role. That being said, there really is something for everyone. The damage you take may be temporary if the Charm can be removed, or it may be permanent (generally the eye Charms), and may also require Strain to power it on top of the Blood Magic Damage. These Charms are not cheap, but can be exactly the thing you need to come out victorious (hence, Desperate Blow/Spell).
Any Group with a Nethermancer, or accomplished alchemist, is going to have access to Blood Charms by simply making them. With downtime, they can create them at half-price, which means if your Group is anything like mine, there is going to be a sack stuffed with Absorb Blows. With even more time, they can also develop their own Blood Charms, which is awesome. The guidelines on doing that are broad, but straight-forward - one of the Blood Charms that I created for my players is the Blood Tattoo.
It is a variation on the Karma Charm (not to be confused with the Blood Karma Charm) and was a gift for providing a service to a Cathan tribe. In this particular game, Cathans are some of the premier Nethermancers in Barsaive. Any character can only have one and it is an expression of who they are - the end result is a totem that represents that Namegiver (typically an animal, but not required). It has a cost of 4 permanent damage and for 1 Strain the adept can spend a Karma on the Talent associated with the Blood Tattoo. This is slightly more powerful than the standard Karma Charm, given the limited access it is something that gives each of the characters a distinctive "you had to be there" story.
Some Thread Items may require this kind of blood magic for a deeper bond with their wielder. Those Deeds tend to be accompanied with a significant benefit of some kind, as well as signs of sentience from the Thread Item as it draws power not just from raw magic, but also from life and that connection. It always goes well.
Probably the most ubiquitous form of blood magic to the denizens of Barsaive, Blood Oaths represent a very serious way to enforce behavior. They are also the foundation of an Adventuring Group. Whether they like it or not (depending on which end they find themselves), these are going to show up. There are three types of Blood Oaths: Blood Peace, Blood Promise and Blood Sworn.
Blood Peace is the Oath that most players will enact, since it is required to form an Adventuring Group. It's pretty simply: not to hurt each other, or allow the other to come to harm through inaction. This isn't one of those things where you can argue your way out of it; it is powered by your life and intent matters. The duration is for a year and a day, and if you come out the other side, you gain a +1 to your Death Rating (unless they are a part of your Group) and your scar turns gold or silver. Breaking this Blood Oath, like any Blood Oath, is bad business. Permanent damage and a Blood Wound. Those take a year and a day to heal, and generally give the impression that you aren't to be trusted. The second part of the Oath is the truly dangerous part and this is not something to take lightly.
Blood Promise is generally the Oath that most players will find inflicted upon them (though some may use it to their advantage). That being said, there is some upside for the dangers and 4 Blood Magic Damage: +1 to a Talent or skill for the duration (up to a year and a day). This is generally used to enforce some kind of behavior, such as not talking about an event, or performing a particular task. It can be an interesting way for two characters to help each other out in the joint creation of a magic item, trying to eke out every bonus they can (see Strain below).
Blood Sworn is the rarest Oath and one that I have only seen once, though also by far the most powerful. It is Blood Magic marriage, essentially, only without the necessity for romantic components. This Oath binds two Namegivers together for a their lifetimes and the effects of breaking this Oath are very rough. The benefits to swearing it, however, are significant. It simply requires a level of trust and willingness to be together for the rest of your life that few without romantic leanings are going to undergo. The requirement of being loyal to each other for at least three years prior to the Oath remove any fears of Vegas-style Blood Sworn.
It is worth noting here that obsidimen have a connection with the rest of their brotherhood that means any Blood Oaths they engage in apply to all of their brotherhood as well. Ergo, they are unwilling to take part in them. While this is an interesting bit of flavor (even appropriate), it presents a number of setting problems: how are they going to take part in a task where the employer requires them to swear a Blood Promise of secrecy? They may not be a part of an Adventuring Group (which is a thing). In the end, I do not like this setting element and have never enforced it in any game. It is limiting with no payoff, it really isn't fun.
While not the most common overall, it is the most common type of blood magic for adepts. This diverse category includes Strain, pushing Talents, ritual blood magic, Dying Act, Dying Curse, Dying Legacy and Dying Oaths. I know, diversity when four of the seven categories include "Dying" is questionable at best.
Strain and pushing Talents are common effects. The latter tends to show up the most when involved in activities that happen during downtime, such as Item History, Forge Weapon and crafting magic items. A +3 bonus for damage that can easily be healed over the course of a day is a strong benefit. The +7 bonus is significantly more problematic and I don't believe I have had a player use that effect outside of combined with a Dying Act. Ritual blood magic is, let's be honest for a moment, usually the realm of plot. While I have had players consider engaging in it, none of them did so in the end. There is this Nethermancer in my current game and he could go either way at this point...
Dying Act essentially allows you to dump all of your remaining Karma into a single roll, in return for dying afterwards. You may as well push your Talents while you are at it - you are already at the worst case scenario. Dying Curse allows you to, quite literally, get in the last word and screw someone over from beyond the grave. On the whole, this effect should be used sparingly from NPCs, but it is a fantastic way to have a recurring NPC go out. Dying Legacy is how many Legendary Thread Items are created and I have always considered it to be an additional element to a Dying Act (or Dying Curse, if that's how they roll), rather than a one or the other situation. Dying Oath is almost always related to a plot; the dying character extracts an oath from another character to do something, and in return they get the dying character's remaining Karma which may only be spent on all tasks related to the oath, even if you cannot normally spend Karma. You have a year and a day to complete the task and if you don't, you cannot regain Karma for another year and a day. I would highly suggest that you intended to do this thing in the first place.
For my own game, I have added Blood Gems, the crystallization of blood spilled during Sacrifice blood magic. They have appeared a few times and they players are somewhat wary of them - while useful in creating magic items, there is the natural hesitance regarding using blood magic to power things. There may have also been some bad experiences with blood magic. The ambiguity surrounding how they are created is part of their mystique: whenever they are found, it is clear that something happened there, something where sacrifices were made. Though there is no indication as to what happened. Were they formed as a part of a Dying Act, Dying Curse, or a terrible blood magic ritual? No matter what, they are significant.