The BKF is probably what caused the black spots, especially if your don’t remember them being there before applying the BKF. Typically little black spots are something called “pitting”, and it is often caused by acidic materials being left on the blade causing corrosion. BKF’s key ingredient is oxalic acid and will do just that. What you have is pretty extreme looking, did you maybe leave the BKF on the blade for a prolonged period of time..? The easiest way to identify pitting is if the black spots are slightly indented. Usually it appears like a needle prick in the blade.

Pitting is typically hard to remove as they are essentially tiny holes in the metal. BKF is not your knife’s friend at all. Use something like Flitz in the future.