Each coat of blue requires applying the rusting chemical, letting it set two hours in the humidity box, boiling the rust to convert it to black oxide, letting cool, and carding off, or polishing each coat. Actual labor for each coat is only one hour, we do not charge for the two hours setting in the humidity box. So we have about 4 hours clean up, and pre-blue polish, plus 6 hours rust blue. Remember this is on a gun in good restorable condition, a gun without any serious problems. On top of this we have assembly, packing and shipping back to customer.
Assembly can be difficult on some guns, due to new blue tightening up clearances.
If you don't want the blemish to be visible, you polish it out, because the blue will not hide it. Therefore many of my blue jobs do have some small blemishes showing through the blue, because the owner does not mind them, and it does not make good economical sense, to do all the pre-blue polishing labor needed to remove them. And some blemishes could be so deep, that to polish them completely out might make the guns remaining structural integrity questionable. If you take every single little blemish out of wood or steel, on a gun over fifty years old, the gun does not look real, a gun that has seen even limited use that long would have a few small dings and dents in wood and metal. Hope this answers some questions, and explains some concerns.