My old fade-to-black routine from the Code Archives:
http://www.blitzbasic.com/codearcs/codearcs.php?code=1281
(Also does a fade-to-white, and a smooth cross-fade from one image into another)
http://www.blitzbasic.com/codearcs/codearcs.php?code=1281
(Also does a fade-to-white, and a smooth cross-fade from one image into another)