Bruce officially took over at the JJB on Monday and always wanted Black to join him.
Asked if it could be his most significant signing, Bruce said: "Of course it is. The most important person is your right-hand man.
"It shows a loyalty to the manager that he has followed me. He said from day one he would and he never waned from that so I'm absolutely delighted he is here.
"You need him to bounce things off. Technically he is very good and he analyses my weaknesses and he is good with computers - I struggle to use my mobile phone.
"He's into the science and fitness side and that is his department and he does it very well."