After much hunting and sighing, I finally found a bottle of the Barry M Instant Nail Effects 'polish' last weekend. It's very weird. It's a matte black formula and applies thick, a little bit like tar. I applied it on top of OPI's Barefoot In Barcelona, and then put a coat of Seche Vite on top. I like this, but mainly for the novelty effect. It's weird applying a layer of black over your colour and then seeing it instantly dissolve and separate across the nail. It dries completely matte, which personally I don't like, but the Seche Vite sealed everything in and left it nice and glossy.
I got lots of compliments - everyone thought they were falsies. I wouldn't use this over any other colour than nude, because it wouldn't suit me, but I've seen it over fluoro pink on someone's toes and that did look fun.