Drymarchon melanurus Central American Indigo Snake
Some other names for this species:
Black-tailed Indigo Snake, Tropical Indigo Snake
Subspecies I've seen:
D. m. melanurus
Central American Indigo Snake
Drymarchon melanurus melanurus Central American Indigo Snake
Chaa Creek, BelizeApril 11, 1998
I saw just the last third or so of this largish snake as it slithered away. The black tail made it very easy to identify, though.
Printed references: