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).
Silver is the ideal color for hiding dirt. I had a silver car once. Unless you drove it down a dirt road during a rainstorm, you basically never had to wash it.
They never did catch you riding dirty.
I don't know why someone downvoted you. For an offtopic joke, this one is pretty good.
I found that having a positive pressure setup with an intake filter practically eliminated the dust buildup on my fans.
If it were about performance and not marketing, they'd try to optimize for resistance to dust adhesion and resource consumption: energy, cost, durability, etc.