I've seen very dark blue that looks close to black, and I have some dark green Nessies that look close to black. Chocolate rose/diamond is actually very dark chocolate, and occasionally a carbon is fully covered that has coloration close to black, but a solid black strain doesn't exist that I've seen (in the USA anyway.)
Overseas I've seen a fire black strain which looked to be a real black...
But things always pop up to surprise me with shrimp, so maybe there's a small breeder who has developed a strain I don't know about yet.