>>8499as the other anon pointed out, it makes some statistical sense to group the population like that, given they have some similar indicators.
there is also some semantics. PRETO is the more objective word for the color black, as NEGRO can be associated with a more poetic sense. It doesen't have any of the negative conotation as in english (think of the country, MonteNegro)
Black Hole = Buraco Negro
Black Pearl = Pérola Negra
Black Board = Quadro Negro
However, the current more acepted word for african brazilians is PRETO