On my latest trip to Japan (September 2019), Dai-Shihan Toshiro Nagato answers the question regarding black belt.
"Q. When should someone be promoted to black belt?
A. Everyone is different. There is no set time. When Hatsumi says something, you say, "Yes Sir!" If he says something is white when it's really black or black when it's really white, you say, "Yes Sir!" In the old days, it was much easier to be promoted. There were no coloured belts at all. You got a black belt on your first day. I (Nagato) was the fastest person to get to 5th Dan ever. In Japan, it is customary to give black belts early to encourage people. You should err on the early side as opposed to too strict. It's ok if someone refuses a rank. I have approached people to promote them and they said no, which is ok, but generally you should say yes and accept it with gratitude. People with a good heart should be promoted faster, good people in the Bujinkan. People with a bad heart should not be promoted, but really, they should not be allowed to train. A guy came to Japan as a 1st or 2nd Dan and was promoted to 6th Dan (though he had to go through 4th and 5th, since that is a major stopping point). He had been kicked out of training by his instructor back in the US, and that instructor ended up murdering his own wife (unrelated to this guy). Try to promote and reward people of good heart first, and try to promote a little bit early."