I have seen plenty of black belts who didn’t have a clue.
Almost every profession includes clueless practitioners.
The tools in Six Sigma are not easy to understand and not easy to learn to apply correctly and well. I would say that a significant percentage of black belts are people chosen by their companies for political reasons, and simply not qualified intellectually to be black belts.
But just as I don't condemn guns for the actions of criminals, I also don't throw out Six Sigma because it's applied poorly by some people.