two brown gerbils were mated and some of the progeny were black. If the same two animals were mated on several

Answer 1

An agouti (brown) gerbil has a dominant "A" gene so is AA or Aa on the first color dimension or loci. A black gerbil has two recessives on this loci (aa), so if two Agoutis both carry the "a" recessive they can produce aa (black) offspring. For more gerbil genetic basics see:
http://abcgerbils.com/genetics.html

Answer 2

You didn't finish your question, but from what you said I think brown is dominant over black, therefore all black gerbils would be purebred. Some brown ones would be purebred and some hybrid. The two parents are both hybrid, or they couldn' produce any black offspring.