Do you call a black belt, Master < last name > or Mister < last name > ? What's the protocol?
I call a black belt Mr. / Ms. <surname> - the same way I do any other rank, whether senior or junior, from white belt on up. Why would I expect students to address me more courteously than I address them?
The only exception is those practitioners VII and VIII Dan (Master) and IX Dan (Grand Master). Except for opening and closing ceremonies, no other titles are generally used; in opening and closing ceremonies, when students bow to the instructor, Sah Bum nim gae (face the honored instructor, bow) is used up through VI Dan; for VII and VIII Dan, Sah Hyum nim gae (face the honored master instructor, bow); for IX Dan, Sa Song nim gae (face the honored grand master instructor, bow). We also use Kwan Jang for our grand master, as he is the head of our kwan (family, group, whatever similar term you wish to use).
As has been mentioned in previous threads, Bu Sah Bum nim is a generic term for I-III Dan, but translates directly as assistant instructor. If a person is the instructor of a class, s/he is addressed as Sah Bum nim - not Bu Sah Bum nim - regardless of rank, because s/he is the instructor. If the person is an assistant to the instructor, then Bu Sah Bum nim may be used for bowing - but in general, with the exception of opening and closing ceremonies, all students below VII Dan are addressed as Mr. / Ms. <surname>.