Why are black cats black?

Black cats are black because of a combination of the genes that the cat has (its genotype). The expression of these genes in the cat's appearance is called the "phenotype".

The black cat has a dominant form of the brown gene. This controls how dark the pigment, eumelanin, is in the hair strands. The dominant "dense" gene dictates that the pigment covers all the hairs of the cat. The agouti gene is mutated to cause the pigment to be throughout the hair strand and not banded as is normal.

What is the percentage of black cats?

Black cats are more likely to be euthanised at cat shelters because they are less popular than other colors. The black cat is 2/3rds more less likely to be adopted than a white cat. Black cats are healthier and the black cat needs to be rehabilitated.

See black cat coats - this expands significantly on the above.