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When powder charges start to erode the metal surface, inside the bore, it hapens most intensly nearest the chamber where the heat from the burning propellant's the hottest - but it occurs throughout the bore.
A "dark bore" means the barrel has started to erode from the wear of the powder granules (which are a little like sand, and the friction of the projectiles, but not enough yet to actually rust or to pit underneath some rust.
The "pitting" is the pinholes left after the rust has been removed from the barrel - or any other surface, for that matter.
The darkness in a "dark bore" comes from the rough bore's failure to reflect as much light as a "bright, shiney bore".
You may also come across the term "ringed bore" - more common with rimfires - that looks like a circle around the other end of the bore opening when the bore is looked through, that can be almost anywhere in the bore (front-to-back) - usually caused by shooting through a bore obstruction, like a previous squib load, where the bullet never made it out of the muzzle.
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