Although true solid black is often desired in breeding programs, black cats sometimes have underlying tabby markings. When exposed to sunshine, some black coats develop a rusty tinge. In the colorpoint pattern, the black gene is manifested as dark brown and is referred to as a seal-point. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.