I think black is fine year-round, but like Cream said, it can get hot if you're outside for a while!
I think the "No white after Labor Day" rule applies to more dressy fashions than casual ones, too. I don't think most people would have issues with a white tee shirt with your jeans, for example -- but if you're going to a dinner party in October, a white dress would be inappropriate (unless it's for sorority initiation or you're getting married, of course!
).