In the musical Showboat, a mulatto singer is passing as white. A rival outs her and the police come to arrest her for miscegenation, sex between the races. The mulatto woman’s white boyfriend secretly pricks his finger with hers and swears to the cops, I have Negro blood in me.
I get the picture.
Being technical again, I guess if blood COULD actually be transferred in that fashion, it would only go in one direction; FROM the person with higher blood pressure, TO the one with the lower; and THAT’s a stretch.