"Raduff got a Blooder in the head because old Ugga wasn't quick enough with his club."
-- Goodman Kneen, in a letter to a cousin (QA3)
The word "blood" is defined as the red liquid that circulates in bodies of humans and other animals (Oxford English Dictionary).
Blood is likely to be what you get when a Blooder hits you during one of these early Quidditch matches, as they tended to be enchanted flying rocks! (QA6)