I saw him from a window and quickly mounted my 300mm lens on the camera. I got one shot before he took off, giving the first clue to his identity. He was about four feet long and could have been any of three species of black snakes we have in this area. But being extra skittish pointed to a black racer (Coluber constrictor).
He paused long enough for another shot, confirming his identity by the white scales under his chin. Racers are nonvenomous snakes that eat a variety of small prey, including rodents, which make them valuable animals to have around. But don't try to make a pet out of one. They will lash around, bite hard and frequently, and defecate material enhanced with especially foul-smelling chemicals when handled.
In this shot, taken after he had decided I wasn't much of a threat after all, one can not only see the white chin, which differentiates it from a black Kingsnake, but also that the scales are smooth, which differentiates it from a black Rat Snake.
I haven't seen him since, but I hope he found our place to his liking and decided to live here.