The best way to fake a bloody wound
is to use your own blood.
THE BLEEDING THUMB. This is a very effective trick, but I am afraid, unless my pupil has some little self-sacrifice, and does not mind enduring a trifling pain in order to amuse his audience, this trick will never be performed. Previous to doing it, you puncture your thumb with a needle in one or two places, near the nail. You then assert that you will cut open your thumb and instantly heal it. You take a handkerchief and tightly bind the thumb therewith, keeping the thumb perfectly straight. You ask for a knife the sharper the better and, having obtained one, you pretend to cut the thumb, which you bend. This causes the blood to flow from the punctures. The blood spreads along the knife, which looks as though it had cut almost through the thumb. You then wipe the blood away, straightening the thumb, and show that there is no sign of wound or blood.
From the 1876 book Modern Magic – a Practical Treatise on the Art of Conjuring by Professor Hoffmann, a lawyer, writer, and magician.
Modern Magic, Internet Archive version>>