seems like someone had already files a bug related to this. but, it seems like this problem still exists.
my video card is a radeon hd 6310 on e-350 apu.
i don't know if it adds information or not, but it is interest me that the black dots only appears only if X is on tty2.
the black dots could easily disappeared if I changed to another tty and back to tty2 again.
it's not that big problem, but I just envy another distro which doesn't have this tiny bit of glitch.