The blood in the original psycho film is in fact chocolate syrup

The soundtrack of screeching violins, violas, and cellos was an original all-strings piece by composer Bernard Herrmann entitled "The Murder." Hitchcock originally wanted the sequence (and all motel scenes) to play without music,^[13] but Herrmann begged him to try it with the cue he had composed. Afterwards, Hitchcock agreed that it vastly intensified the scene, and he nearly doubled Herrmann's salary.^[4][14][15] The blood in the scene is in fact chocolate syrup, which shows up better on black-and-white film, and has more realistic density than stage blood.^[1] The sound of the knife entering flesh was created by plunging a knife into a melon.^[16][17]

^{http://en.wikipedia.org/wiki/Psycho_(1960_film)}