The flicker is likely to be because the blits haven't finished before the frame is flipped. Are you flipping the frame after a VWAIT, or from an interupt?
In any case I've never had much luck with Blitz's built in Octamed support, I strongly recommend Leffman's OMED library:
http://eab.abime.net/showthread.php?...ge=3&styleid=1