A dark bore (in my experience) may or may not indicate the ture sondition of the bore, or even how well it'll shoot. I've had some that (milsurps) that were really bad, once cleaned, had good rifling, and decent accuracy. What I've found to work really well (after spending HOURS scrubbing bores) are some of the new(er) Foaming Bore Cleaners. Worth a try, and you may find that the bore isn't as bad as you might think.
I'm "cleaning up" one for a friend (its and heirloom, she has no intention of shooting it). In this case the bores pretty good, but the outside is rough (and its actually made up of 2 different rifles----since its a "take-down", there are 2 serial #s!) But I found, when I researched this gun, that they made them with a variety of options, and SOME can be worth a quite a few bucks if they're in really good shape.
These were often called "gallery guns" as they used them in shooting galleries. The one I have here has a more triangular shaped loading port (to speed loading from a tube) not so rare, and a pistol grip stock (seem to be much rarer).