since the update to vray 3.4.04 (but also before this release) I have dark corners… this was a problem in the early versions of vray… long time ago… 
now I don’t know if this is really a new phenomena, or if it depends of the scene…
I think it’s really there since vlado have made the fix for the noise problem when subpixelmapping is on. I render with BF/LC always. (finally. :))
best regards
themaxxer