In article ,
Phil Addison writes:
Blood:
- soak in biological washing powder in cold water (I think! not sure)
yes (probably). also if you just rinse it off before it dries it wont
stain in the first place.

Plain soap and water will work for a while even after it dries.
However, eventually the red blood cells break down releasing their
iron, and that's much harder to remove. The thought just occured
to me (although I've never tried it) that a rust remover such as
phosphoric acid might work once that's happened.

--
Andrew Gabriel