Why should I buy a barrel of blood?
As of now, there isn't any in-game use for these other than selling them or general Armok-pleasing.
I also don't think that there's any way to collect them. I thought that this was what extracting at the butcher's shop did, but according to the wiki, that's just for venom and liquid fire.
Hopefully there will be some use in the future though like transfusions, occupying night creatures for a while, or coating your fortress for decorations, but alas, this does not exist yet.