DON'T USE A STEEL BRUSH. This will scratch and possibly do serious damage to the bore.
Often a dark bore is is a bore that's covered in tiny pits, giving it the dark look.
You see this in a lot of older military firearms that were shot with corrosive ammo, and it caused the tiny pitting.
In most cases, you can't shine the bore up since it has a surface that looks much like it's been bead blasted.
A dark bore is usually "forever" because of the rough surface.
Since in order to get the surface smooth you'd have to remove significant metal, this would badly oversize the bore.
About the only thing you can do is buy some JB Bore Paste and a can of Kroil from Brownell's.
JB Bore makes a standard cleaner and the Bore Shine.
The cleaner paste is intended to REALLY clean the bore, the Shine type is a sort of finishing step to help shine it.
The Bore Shine will likely do you no good at all, since it's intended to help shine up a GOOD bore that's already shiny.
Use it according to the label directions.
One other possibility, but one that may be bad for the gun is to buy a "fire lapping" kit.
These kits have abrasive coated bullets that are fired down the bore to lap out imperfections, and in the very fine grits can polish the bore some.
These kits usually require that you reload the bullets into your cases and use special light powder charges.
These are usually made for rifle calibers, and you'll have to search for a company that sells a pistol type kit.
Bottom line: Other than normal cleaning and possibly the JB Bore Paste, you just have to live with a dark bore.
This does NOT mean the gun won't shoot nice groups.
Many guns with dark, pitted bores shoot very good groups.