To be honest I think it is an optical thing most of the time. IE your cat was never really black but the usual lighting doesn't show the true color.

But cats that do spent a lot of time in direct sunlight may get slighter lighter brown with time.

I couldn't find any study or data on this so it may also be a cat legend :p

I know my cat is very much light brown in the sun even though she doesn't spent that much time under the sun.