It could have been a poorly stuck 9 or it could have been buffed out during a reblue.
Oh, you already said that.
Seriously, no way to really tell without pictures. Look for other clues. Check out the rest of the letters on the gun, any other signs of a reblue? Any other splotchy weakening of the blueing, or pitting underneath the current blue? All could be signs of a reblue job. You should be able to determine fairly easily. If not, probably just a poorly struck 9