On the two occasions that I have seen black snakes at WDW, they looked like Racers to me. They are slender black snakes w/ white chins, they tend to range from 2-5 ft. long. Racers are not poisonous, but if you harass them or corner them, they can bite. It's about the same injury level as a bite from a kitten, they have a couple of sharp little teeth.
Mostly I've seen little green garter snakes; those are totally harmless unless it's possible to be gummed to death.