Is six sigma black belt good to have for promotion?

Absolutely. Not to mention that it will help you on the civilian side too. Program Management is another great cert to earn.

Six sigma is not only a good thing to have to help for promotion but whenever you make the decision to transition to civilian life it will give you a good foot hold for employment on the civilian side.

Yes definitely any form or level of education is a big plus towards promotion, specially this type of certification. Don't forget to iPerm your certificate if you have not done so. Good luck.