A long time agao, and I am ancient, I saw a film, possibly in black and white, with a blonde boy whose rich dad brings home a young girlfriend after his wife, the boy's mother, dies. I remember the boy trying to manipulate her, a scene where he is in the bath tub and she answers the phone or visa versa and at the end either he kills her in a car accident by mistake or visa versa. I remember it was quite controversial at the time as I think the boy blackmailed the dad's girlfriend to take off her clothes.

This is in color, and it isn't all that ancient, as I was in my early 20s when I saw it. Perhaps I have the wrong film.

What the Peeper Saw (1972)
Aka Night Hair Child
IMDb review (edited):
1) This is just what the world needed: The star of "Oliver!" as a pubescent sex pervert and psychopath. A young woman (Swedish sex symbol Britt Ekland) marries an older man (Hardy Kruger) soon after his first wife dies in a mysterious bathtub accident. All is fine until his creepy 12-year-old son (Mark Lester) returns home early from school. The cold, precocious youngster tortures and kills animals, tells malicious lies to turn her husband against her, spies on them in bed, and, for the piece de resistance, forces her to strip while he reveals the truth about what happened to his mother....
2) The scene in which the boy imagines his mother's corpse (whom he has callously killed in the film's very opening scene) in the pool is effectively macabre; the finale, too, is worth waiting for: The boy almost coerces his stepmother (no sooner has she been released from the asylum) into murdering his father and becoming his lover since he tells her she's closer to his own age (he being 12; she, 22; while his father is 42); she gives him the impression of agreeing with this latest scheme of his but, realizing the kid will never change, immediately provides herself with the opportunity to get rid of him once and for all…

What the Peeper Saw (1972) - IMDb

Since it's been more than 40 years since I've seen it, I'm hazy about the ending. It seemed to be rather tricky because of the other fantasy and dream sequences.