Not sure I’d call the Black Mountains a small area myself, but anyway if you already know Cwm Banw, try Tal y Maes bridge just up past the Hermitage, good wild camping spot and fairly sheltered. Mynydd Bychan is also a nice spot, is it primarily the Black Mountains you are looking at? Another lovely spot is just past Blaen Bwch, take the lane from Capel y Ffin past the Grange to its end, follow the path for approx half a mile and look down to the stream Nant Bwch on the left.