Another bit of "Psycho" trivia: Although the slow pull-back from Janet Leigh's open eye after her murder creeped out many viewers, at least one proto-nerd (a doctor) wrote a letter to Hitchcock in which he pointed out a problem with her pupils. Pupils dilate at death, and hers were shrunken to pinpoints under the bright studio lights. The doctor suggested that in the future Hitch apply belladonna drops to the eyes of his "victims," to dilate the pupils. The director took his advice, probably most noticably in "Frenzy."
Two reasons for the chocolate syrup "blood": One, although MPAA ratings didn't exist yet, and Technicolor red "blood" had appeared in movies before (An example of a kid-appropriate film to do this would be the delightful 1940 "Thief of Bagdad," which contains a brief shot of a guy with his cheek slashed open in a sword fight), the context and quantity of blood would have pushed "Psycho" onto the Censorship Board's black-list had it been red. Two, some B/W film stocks don't translate certain colors well. Fake blood thinned in water into a pale pink, nearly invisible on film; chocolate syrup provided better contrast.
Hitchcock also had someone stab different kinds of melons until he heard the sound he wanted and went with that one.