"Blood" is obtained by putting mobs or players in a Smeltery while there is a molten substance in it. With players, it damages 2.5 hearts at a time and yields 5 mB of Blood. Rotten Flesh can also be melted down in Smelteries for 5 mB of Blood. The amount can differ if the modpack you are playing changes it.
It is used in making Pig Iron and Coagulated Blood.
Coagulated Blood is made by pouring 160 mB of Blood into a Casting Table. It can be eaten to restore 1 hunger and give 15 seconds of Health Boost. Health boost allows you to slowly generate extra health past your 10 base hearts, but will take the extra health away once the effects wear off.
When placed on a Drying Rack will refine it into a Coagulated Blood Drop.
Coagulated Blood Drop
The Coagulated Blood Drop is a more refined variant of Coagulated Blood, it is a food item that can be eaten to restore 3 hunger, as apposed to Coagulated Blood's 1, and gives two and a half minutes of Health Boost II. Health Boost II does the same as Health Boost, but it only takes away half of the extra hearts when it runs out.
Blood Channels are created by placing four Coagulated Blood into your crafting table with one redstone. They are a half slab-sized block of flowing blood that flows away from where you were when you placed them, but they will not spread beyond the one block that you placed them on. They can be picked back up and placed again, making them useful as conveyor belt-like blocks, allowing you to transport items, mobs, or players with ease, and without having to worry about water going all over the place when you take things apart.
Bucket o' Blood
This is exactly what it sounds like, a Bucket of Blood. You can get this by using a bucket on a tank containing at least one bucket's worth of blood, or by putting an empty bucket on a casting table and pouring blood over it. This can be placed and picked back up. It flows like regular water, flowing 7 blocks beyond the source block, and will flow into the first avalible hole or lower elivation. It is no harder to swim through than water, allowing for an asthetic use in places you need flowing water but don't want Blood Channels.