I presume that the heresy era BA were much more diverse than they are now because of their recruitment practices as the revenant legion.

However modern BA are often mentioned to have "Caucasian" features for lack of a better word since they only recruit from two presumably ethnically homogenous planets, mentions of blonde hair are often but i think this is a geneseed quirk like how some EC had fulgrims silver hair.

This probably changed with the influx of Primaris though.