When grading a rifle bore, dark usually means that the surface of the bore does not shine. If it does not shine, that means that there is debris in the barrel. This debris could be any number of things including fouling or dirt. But this could also indicate pitting caused by corrosion. The only way to confirm actual corrosion would be to scrub the bore down and make an attempt to clean it up.
It's not to say a dark bore won't shoot well. The lands are what affect accuracy the most. But for the most part, a dark bore is negative characteristic from both a shooter's and collector's standpoint.