I discovered this mapping website while training for various events in 2008. I've no idea how accurate it is as I've no GPS to campare it to. I still find it useful as a yardstick and think it's accurate enough for my needs.
Here is the route I took for my Black Mountain End 2 End.

I'm publishing this to compliment my blog, it is not intended as a route map for you to follow. If you intend to walk a similar route I suggest you fully study the Ordnance Survey OL12 map of the Brecon Beacons National Park, Western Area.
You should also be aware that I have many years of experience and training in this area and I know only too well its changeable weather conditions.

Make sure you are capable of walking and navigating in such an area, being suitably equipped and prepared for all eventualities. Mobile phones work sporadically in this area - do not rely upon them!!
Consider any personal emergency to be your own fault for not reading your situation properly and be ashamed of yourself for endangering anyone who may be needed to come to your aid!!

Rant over - get yourself prepared and get out there and enjoy the Black Mountain as much as I do.
Feel free to email me or leave your comments, I will reply to all.

Diolch yn fawr, Lee.