If it's a concave blemish, and not protruding out into the actual barrel cylinder, the rifle should be perfectly safe to shoot as long as it's not a deep gouge. Before buying, you should ask to fire the rifle to make sure the blemish isn't having a harmful effect on accuracy. If it were me, I'd probably just set up at 50yds real quick, and just see if you can get any sort of consistent group. You don't need to worry about the sights too much (if you're a little off from where you aimed, the sights can be adjusted later), you just want consistency to eliminate the idea of the blemish causing problems. Make sense?