Feline GenesCats' bodies, like humans, are made up of genetic materials, called genes. Genes and their associated alleles are responsible for the color of our eyes, the color of our hair, and other inherited physical traits from our forebears, such as long legs.
MelaninMelanin in Cats' Hair is the stuff that determines the difference in colors of eyes, skin, and hair. Melanin in the hair shafts *takes the form of microscopic granules, which vary in shape, size, and arrangement, giving a variety of colors.
Recessive Genes in Cat HairGenes may be dominant or recessive. A very black cat may appear to "rust" in the summer sun, possibly the result of a recessive red gene. I've observed that this seems to be more common in longhair black cats.
You may also have seen black cats with white roots, which are called "smokes." Longhair black smokes are very beautiful when they move and their white roots flash before you.
To make the topic even more complicated, it is thought that the original color pattern of all domesticated cats is the tabby. Therefore, all cats today (with a couple of exceptions), carry the recessive gene for tabby. However solid colored cats, including black cats, have yet another recessive gene that suppresses the tabby pattern. If the tabby pattern is not completely repressed, you may see a hint of tabby markings in a black cat in bright light. The tabby "M" is the most distinctive of these.
I think I've given you enough information today to get the general idea of why all black cats do not always look alike. You are encouraged to look at the sidebar material to research more information.
* From Fanciers.com article on Cat Color Genetics