But there's no reason you couldn't use it as an ultrablack pigment layer and simply tolerate the presence of some highlights. Wouldn't even have to be 'matte' s

But there's no reason you couldn't use it as an ultrablack pigment layer and simply tolerate the presence of some highlights. Wouldn't even have to be 'matte' surface, you could put a high gloss surface on there and still have it be different. Because it isn't just black paint, because it's a sponge-like structure inside a medium, that means you can use it as a simple pigment in a normal paint job, and it would be blacker than normal pigments in that context. Might be possible to make an 'ultra gloss black' in that way. Highlights would reflect, but anywhere that isn't a highlight would be the depths of space. The glossier, the more extreme the effect.