MacBook Pro detects external monitor but shows black screen

Question

MacBook Pro 8,1 (13", i5 2.3Ghz) - when I connect an external screen through a MiniDP -> DVI adapter the Mac detects it (knows it's a Samsung SyncMaster and lets me change its resolution) but doesn't show anything on screen. Even though the Mac thinks that the screen works properly (I can move the cursor out of the Mac's screen), the external monitor's led blinks as there was no signal at all. Changing screen resolution doesn't help.

I've browsed many similar questions on the internet, unfortunately most of them suggest upgrading the OS. I'm using Mac OSX 10.6.8 Snow Leopard. What should I do?