Is it OK to go to a K-pop concert in Korea alone as a foreigner?

Yes, it is common and safe. Plenty of fans fly in and go solo, the venues and trains are safe late at night, and staff and other fans are generally helpful even with the language gap. The main friction is English signage being thin, so lean on Naver Map (Google Maps directions are limited in Korea) for getting there and home. Standing alone in the pit is normal too.