Well, I have a bunch of lower-end black fans, some of them quite old, from before transparent cases were a thing. They're actually pretty much gray if I don't wipe them off.

Noctua's signature... brown-orange? Whatever that color is, it has the same issue. The blades are basically gray if I don't wipe them.

Haven't seen anybody start a gray craze, though. Though I have a grayish motorbike that also shows dust and dirt like nobody's business (it's a bike I use strictly on paved roads).