Wow, that's a night and day difference!! I've had kittens that were born white and turned out solid black and kittens born black that lightened to a grey, but never an adult kitty turning that dark so drastically. How old is s/he? Have you had her long? Maybe it's a seasonal thing.

I know with Aussies, the merles tend to get darker with age as Charlie is proving (he's a LOT darker than he was a year or two ago), maybe with certain kitty coats its the same?