Streaks on #2 common red oak

Q: About 3 months ago I had a #2 common red oak floor sanded, stained with natural (same color that was originally used) and refinished with 3 coats of Swedish finish. Now there are many uneven dark (nearly black) lines that are unattractive. It may be my imagination but it appears to be getting more dark streaks every day.

Was something improperly done with the staining or refinishing of the floor?

A: #2 common? No doubt that wood is full of dark “mineral streaks” and knots. All normal and part of the grade of flooring you have.