I don't know the answer to this, myself. So I did a bit of searching for you and found a DLL userlib that extends Blitz's OS functions (which I would assume includes current resolution and color depth). I hope this is what I think it is.
http://www.blitzcoder.com/cgi-bin/showcase/showcase_showentry.pl?id=Rob__Hutchinson01222002120752&comments=no
http://www.blitzcoder.com/cgi-bin/showcase/showcase_showentry.pl?id=Rob__Hutchinson01222002120752&comments=no