You could easily approach the Black Mountains from the Hay on Wye side, coming down past Capel Y Finn, and its not that far from Brecon.
I know i will be heavily criticised for it, but can i suggest another alternative for real solitude and possibly an area of the Brecon Beacons you have never considered, its a walk of incredible interest, and you will more than likely have the place to yourself.
Do a web search for the Craig Y Nos country park, and ascend the Carmarthen Vans from anywhere along the A4067 Glyntawe Sennybridge rd.
I know you have asked for information on the Black Mountains, but as this is supposed to be a forum offering advice, i will offer it, you can take it or leave it.
Drive as far as Cnewr, and you will see McTurks farm, you cannot miss it, as its one of the largest farms in this part of the Western Beacons, and also another point of reference is the Cray reservoir.
This section of the Western Beacons, also known as the Black Mountain, is an area very few walkers venture near, simply because its an unknown quantity.
A very remote alternative for your intended part of the Black Mountains, is to ascend them from Pengenffordd, that's a real solitude location if there was one, and its not that far from Brecon, but it takes some finding, and is not really a wild camp area.
I would like your thoughts on my suggestion, Carmarthen Vans (Black Mountain) or an ascent of Waun Fach and Pen Y Gadair Fawr from Pengenffordd, (crossed fingers for no mist) as the Waun Fach to Pen Y Gadair Fawr ridge is incredibly challenging in pea soup.