If you are already in the gunsmithing business, then bluing guns can generate extra profit, after the setup has paid for itself. The setup itself, just to get off the ground, with the tanks, chemicals, and polishing equipment, will sit you back around $3 grand, which will take doing about 30-35 guns just to pay for it.
First, you'll need to learn how to polish metal, so it wont look like a sucked-on lollipop. Here, you can get hold of some cheap guns, your's, and maybe some relatives and friends, if you tell them you'll refinish them for free. Don't, however, take in anything expensive at first. The main thing is learning to polish, and there are some videos out on how to do this correctly, along with how to do caustic bluing. Once you learn this, then you can accept guns from customers.
The main problem is getting people to pay you what it is worth, as polishing a gun takes time to do it correctly, and you will have at least a half day labor in a gun, easily. This counts the time for disassembly and repairs, then polishing, bluing, and re-assembly. It does not count refinishing wood, which is another time consuming job in itself.
A small shop should have enough guns coming in to at least light up the tanks twice a week. Any less than that, and it will take a while to start generating a profit.