prof - black paint is typically carbon pigment, which is light-weight as pigments go.
The issue would be added mass proportional to the extant moving mass. I suspect that in a woofer or midrange, the added mass would be functionally inconsequential, but in a tweeter it might matter.
I'm interested enough to try a light misting coat on my CS1.6 woofers. I can sweep before and after and compare output including all harmonics and time domain performance.
I'll use lacquer so I can wipe it off if necessary.