A dippy blonde ditches her cop boyfriend at a restaurant so she can sneak off with another dude she saw there. At his apartment there's a scene of them flirting for about twelve hours before he tries to kiss her, which disgusts her apparently 'cause suddenly she's not into him anymore. So he tries to rape her 'cause that always wins a lady over but she kills him with a cheese knife. Knowing no good can come of this, she spends the rest of the movie with crazy eyes.
Luckily her cop boyfriend is on the case and willing to cover for her. Unfortunately, a weird vagrant with a criminal record saw the dippy blonde with the dead guy and has decided to title of the movie them (for what he never really seems to say).
The problem with Blackmail is that there isn't a single likeable character in the bunch and it's really hard to care about or even pay attention to what's going on when you don't like anybody involved. I sat through the whole thing, but only barely.
Hitchcock's cameo is near the beginning on a subway train. A little kid pulls his hat over his eyes.
End of line.