What is wrong with your cat if it has black drool?

There could be several reasons as to why your cat as black, or dark, drool. Worst-case scenario includes a serious liver disease called hepatic lipidosis. Best-case scenario is something is stuck in your cat's teeth, bleeding gums, bacterial infection, etc.
If possible - without hurting the cat or yourself, that is - give your cat's mouth a rinse and see if there is any visable cause to the problem and take the cat to the vet ASAP.

Do not rely on internet research to diagnose medical issues. Always seek professional advice before coming to a conclusion.