If you really want to avoid blueing and the blueing is in good shape at the dent, a piece of the thin mylar packing tape over the dent will protect the blueing. Then use a freshly polished steel hammer to tap the dent out. Using the tape will require you to hit the barrel harder to get the same effect as without tape, but it will also protect from marking the surface. It really works well.
I got the idea from aircraft rivet tape where mylar tape is used to hold a line of rivets and protect the skin when installing flush rivets.
