I haven’t ordered any black yet but did you try sliding the putty knife between the part and raft? That might work. I have also noticed it comes off easier after it cools down.
Well, it’s a little weird that you’re having issue with black filament. I’ve printed different types and colors of filament, and I did not encounter any problems except some minor issues that I can easily fix. I’ve read from 3d2print.net that getting a good print with different colors depends on how you set up the printer and the temperature settings.