After a successful hike up Mt. Washington, something got the better of me on the way down. I thought the area was just irritated, and didn't think much of it, since by then most everything was a bit sore. The next day I noticed my foot wasn't getting better but worse; the area was becoming very dark and possibly spreading. I called the doctor the next day and he put me on Doxycycline and Rifampin (two apparently strong antibiotics). The infection spread across my foot and a couple of inches up my leg over the next couple of days but the color began to fade. My foot is just about back to normal, thankfully, after reading lots of horror stories online.
One good site I found was http://www.badspiderbites.com/spider-bite-treatment/ which claims that injecting 1/10th CC of Dexamethasone (a cortisone) seems to do the trick. I doubt a brown recluse was the culprit seeing that it was in New Hampshire. My best guess is a wolf spider, but I guess the lesson learned was next time I'll wear hiking socks.