Not all blind people have guide dogs. In fact, most don't. It's just good sense to avoid making fun of someone because they are wearing sunglasses. Furthermore, being blind doesn't necissarily mean that one can't see anything at all. For example, I'm half blind, but if you shined a flashlight in my blind eye, I'd be able to see the light, and I can see some colors as well.

Maybe I'm just extra sensitive about it because I have cogenital toxoplasmosis, and I know a fair number of blind people as a result.

Of course, this isn't the first time the president has made such a gaffe. I don't blame him for it, since it was obvious from the footage that it was entirely accidental. This isn't half as bad as the time he waved at Stevie Wonder. (although I heard that might be an urban legend)