For some reason that part of print was not properly in developer. Maybe some grease (from fingers or like) or drop of sveat from your head or something else, or just you didn't put print properly into developer. Maybe damaging of paper when taking out of packadging, can be many reasons. And as you said you checked negative and negative seems OK, simply make another print. I had simillar probelm about 15 days ago, and in my case paper wasn't properly placed into develping tray. So, I made another print which was OK. No reason for scratching a head, simply make another print...