Hiya,
Wondering if there is any possibility (even with extra cost) of a black soldermask?
Failing this, is it possible to use white soldermask and invert the silkscreen layer, so that the negative gives the impression of white on black?
Cheers 
Black is not provided, but you can make the white for the soldermask.
Hmm, that’s certainly an odd way to achieve what you’re after. I think it would depend on whether your CAD program could generate a Gerber file for this special silkscreen layer. Even then, it would be a pretty funky Gerber file & Seeed might not accept it.