Black Widow feels like cheapish TV filler, the kind of thriller you might catch on a TV station in the 90s that had some open movie slots and didn't want to spend too much money on filling them. It's unremarkable in every way, no twists, no class, just a straight-up plot that takes you from A to B.
Catherine is a young woman who seeks out rich old men, marries them and runs off with the inheritance. She's very good at covering her tracks, until Alex, an eager cop, starts connecting the dots. Nobody believes her though, so she has to find a different way to bring the truth to the surface.
Black Widow is little more than a simple plot coupled with functional direction. The performances are mediocre, the cinematography is bland, the soundtrack a real snoozefest. The killer is pretty boring, the police investigation is formulaic, and the plot is a lazy copy/paste job. At least the pacing is decent, and the runtime is short, other than that this was forgettable genre filler.