"B" key to black in KN 09 and Snow Leopard

I've got a strange bug--in KN 09 with Leopard it was possible to hit the B key to black out the screen and then command-Tab to another application (like safari) without getting a 'flash' as KN switched out of full-screen mode. With Snow Leopard, curiously, this is no longer possible. Hitting the B key does black the screen, but Command-Tab now releases the screen black function, and the flash happens. Has anyone else seen this? Any workarounds? It's an essential trick for slick presentations which involve app switching, and it's a shame to see that it changed.
Thanks