First of all, I have to congratulate you on a great shot. What wonderful timing, and what a dramatic story!! I am definitely envious of your luck.

However, I have to question whether or not that was a black rat snake in your photo. Of course I'm not getting the best look of it, and I wasn't there to see it in person, but the snake sure does look like a black racer.

Consider these diagnostic features that I personally use, although I admit they may not be valid outside of Virginia where I live. The first thing that clued me in that something was different was the large rostral scale. It is larger and considerably more pointed at the top than a typical black rat's. In addition, there is a lack of bold stripes on the edges of the upper labial scales, there appears to be a lack of keeled scales on the neck, the iris of the snake appears to be dark like a racer, and there is a strong "eyebrow ridge" (don't know the technical term) that is typical of a racer than a black rat. Consider the pictures linked below...

Black rat
http://www.pbase.com/rcm1840/image/43914116

Black racer
http://www.cortland.edu/herp/keys/images/snakes/cconstlg.jpg

Neither one of them are the best of photos, but they were best I could come up with without going through the hassle of uploading my own.

Confusing the two species is easy to do, even for experienced eyes. When I was doing a Google image search for black rafts, I frequently came upon black racers that were mislabeled, even on some seemingly legitimate web sites!

bradtort, please post back with additional photos.
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"The irrational fear of snakes is the only excuse a grown man has... to act like a complete sissy" - Colchicine