Yours is definitely a black rat snake and a nice one at that. It's not that difficult to tell the difference between a black rat and a black racer, but it gets much easier with experience. 
Here's how I tell (for adults):
Black rat snake: heavier bodied black snake often with some evidence of a pattern on its back. Black rats usually have a white-ish lower jaw with some white on upper jaw around the mouth. The belly is not plain; most of the snakes I've seen have some sort of alternating black/white pattern (i.e. checkerboard, etc.).
Black racer: slender solid black snake with a solid gray, white, or blackish underbelly. These snakes often have the tip of their snouts colored a light pink and have a light-colored lower jaw. Most of them are pretty nasty if cornered.
Black rat snake: http://www.uga.edu/srelherp/jd/jdweb/Herps/species/USsnakes/elaobsobs15.jpg
Black racer: http://www.agfc.com/!userfiles/int_images/snakes/black_racer_big.jpg
Congrats on your nice find!