thanks. actually, it emerges that there are 4 dungeons in bf2 when i was only expecting 3. i found the first one closest to the village as i was clearing out the map from the south end.
I think there are 6 altogether. The black map thing is a nuisance but it does seem to clear itself when you keep walking. I had the same problem with the mini map etc going completely black from time to time. I also found that when I returned to the BF from the main game that most of the map that I'd cleared would have gone black again.
Fortunately, as I walked across it again it would re-appear in large chunks. In other words I didn't need to walk up and down the whole map again. However, it did mean that I had to try and remember where the heck all those entrances had been located. Maybe it's a good idea to draw a rough sketch of the BF area for each act as you reveal it?
There are a great many little bugs and quirks in BD, but most of them can be ignored or worked around without too much difficulty. It's also worth having patch 1.45, even though it only fixed some of the issues.