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The blue glow, Cherenkov radiation, is something that happens underwater, though. It's not really a drop-in replacement for, say, a puddle of radioactive waste.

I think that the problem is more that artists just want a way to indicate that something is radioactive, but we can't see radioactivity, so they had to seize on some sort of convention that deviated from the real world. It doesn't really need to reflect reality to work, just as long as the convention holds. And the practice of doing that in art isn't that new, either -- think of the halo, which serves a similar purpose:

https://en.wikipedia.org/wiki/Halo_%28religious_iconography%29

A halo (from Ancient Greek ἅλως (hálōs) 'threshing floor, disk'; also called a nimbus, aureole, glory, or gloriole) is a crown of light rays, circle or disk of light that surrounds a person in art. It has been used in the iconography of many religions to indicate holy or sacred figures, and has at various periods also been used in images of rulers and heroes. In the religious art of Ancient Greece, Ancient Rome, Christianity, Hinduism, and Buddhism among other religions, sacred persons may be depicted with a halo in the form of a circular glow, or flames in Asian art, around the head or around the whole body—this last one is often called a mandorla. Halos may be shown as almost any colour or combination of colours, but are most often depicted as golden, yellow or white when representing light or red when representing flames.