Sounds like you're laptop screen has got dead pixels in it.
Not much you can do, its not an uncommon problem, my sister's PC has a dead pixel, except it's always on (white) rather than off (black like yours).
If it really bothers you, then I think the only thing you can do is replace the laptop/ the screen.
Unless I'm wrong and it's not dead pixels... though I can't think of what else it'd be =/