Black is not a color - not for keying, anyway. There's simply nothing where the algorithm would have anything to work with. You will have to use otehr methods to generate a matte liek using channel operations or tools liek the simple color keyer, luma keyer and so on.
As Mylenium says, Keylight is for color keying, meaning using a color like green or blue as the key color. Not black or white.
I am not 100% sure, but would RG Unmult not help as well?