Does Tales of the Black Kingdoms say much or is that just adventures?
I find the info in the Hyboria's Best books (Fierce, Finest and Fallen) to be helpful with ideas on culture and things like that, plus they have some variant rules on alternate optional feats you can give Black Kingdom tribes and mutliclassing options.
I know it isn't Mongoose but the Ultimate Guide to Conan by Dark Horse is good on ideas for better background info.