I have seen it mentioned only once, by DaveC, in this thread and it definately is worth repeating.... Hydrogen peroxide.

Hydrogen peroxide will bubble/foam up when it comes in contact with blood. It' s perfect for those leaves that have red spots, or other confusing nature elements. Squirt it with uber cheap Hydrogen peroxide to see if it is blood or just mother natures coloration.

A cheap squirt bottle at Wal-Mart is like $1 if that, and a whole bottle of Hydrogen peroxide is about the same. Don' t forget to mark the bottle with a sharpie pen.

IMHO, this and toilet paper (for marking found blood and it will decompose) are two things you should have on hand! Better to have and not need then to need and not have

Also, does anyone else stick the arrow right back in the hole you pull it out of? (assuming pass through shot) After I inspect the hair and blood on my arrow I will often stick it right back in the same hole and put a piece of TP on it so i can easily look back at where it was.

Great subject!