Black moor goldfish naturally turn that color as they age. There's nothing really that you can do about it, sorry.
A black moor and a comet goldfish can readily mate together. I have successfully bred them myself in a 60litre tank. However if you are expecting fancy goldfish you may be disappointed. The genes for the black moor are recessive, which means the majority of your fish will be normal goldfish. Most of my youngsters have not yet changed colour yet so I am still waiting to see whether they will all be gold, or whether I will get some black colouration. good luck!!
Short answer: No. Long answer: Because the black moor is a fancy double tailed variety of goldfish, they are slow and clumsy and will not be able to compete as well for food against a single tail speedy comet. They CAN be housed together, but because of the black moor's special needs it isn't recommended.
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