This thought just occurred to me: if dementors are blind, how do they tell one human from another? Oh yeah, as per book 4, THEY CAN'T. All humans look like food to them, just at varying levels of having been munched on. So sending the dementors out amongst hundreds of people (at Hogwart's) to look for one in particular (Sirius Black) is rather a stupid move on the ministry's part. It's rather like asking a completely blind human to feel over 100 pre-shelled hard boiled eggs and asking them to find the one dyed green. They may eventually get hungry enough to pick one at random and call it the green one, which is why, I think, Harry almost got Kissed.