One of the reasons Alfred Hitchcock shot 'Psycho' (1960) in black and white was he thought it would be too gory in color. But the main reason was that he wanted to make the film as inexpensively as possible (under $1 million). He also wondered if so many bad, inexpensively made, b/w "B" movies did so well at the box office, what would happen if a really good, inexpensively made, b/w movie was made.
But thant's not where the use of both black and white stopped. In the opening scene, Marion Crane (Janet Leigh) is wearing a white bra because Alfred Hitchcock wanted to show her as being "angelic". After she has taken the money, the following scene has her in a black bra because now she has done something wrong and evil. Similarly, before she steals the money, she has a white purse; after she's stolen the money, her purse is black.
It's kinda like having the good guys in westerns wearing white hats while the bad guys wear black hats.
Oh and by the way, Norman Bates is number 13 on our "Top 100 Monsters Of Movies And Television" list!