Does this scare you?
Those all two-prong receptacles normally have a little burn on them like that. People seldom realize it, because they are 99% of the time brown colored receptacles. Those have been painted, so the burn shows up better.
Why does this happen? The receptacles are very old, and the contacts inside lose their tension and hold the cord prongs very weakly. They not only heat up during normal use, they can throw out a little flash when you insert an attachment cord. The corrective action, for the immediate time, is to replace them with new 2-prong receptacles. The rest of the home's electrical system is of equal vintage, no doubt, and is pretty much at the end of its useful life. The whole system should be considered for future replacement.