OK this problem is driving me crazy..
this is something that bsplayer team must fix.
a lot of users report this problems on other forums and also here:
http://forum.bsplayer.com/bug-reports-codecs/10265-grayish-black-bars.html http://forum.bsplayer.com/general-talk-support/10026-what-bsplayer-has-become-bugs-really-upset-customer.html
(the user in the second link report some bugs that are not been fixed yet)
CoreAVC H.264 decoder is the most popular codec for HD movies (mkv files).
many users face this problem with bsplayer and coreavc.