Hi,
Dark negs = Overexposed as you said or it could be overdeveloped.
Though not an exact science, overdeveloper = contrasty with lesser midtones.
overexposed, more grey and not contrasty, the picture could be more grainy as a
result and less sharp.
The problem with the above is that most of us turn
on AutoExposure during scanning, making it hard to
evaluate the scan. Furthermore, scannings requires
adjustments in Photoshop, ie. the curves segment.
Making an educated guess without seeing your negatives
or your scan process, I would say over-exposed is more likely.
You have to adjust in PS curves.
cheers!
raytoei