I have a Browning semi-auto that I've had for 20+ years, it's in great shape overall, but the barrel is starting to develop some (shallow/superficial) pits from a combination of dings and saltwater exposure on coastal duck hunts.

My question is whether having the barrel re-blued at a gunsmith's is worth the cost. Will it just gloss over the pitting, or will a good bluing remove them to the point where they won't be "seed" sites for future rust/corrosion?

Bluing does not fill pitting at all. To achieve a factory finish, the metal surrounding the pits needs to be removed to the same level as the deepest pit and polished.

Your best bet is to use some oil and 0000 steel wool to remove the rust. Then just keep the shotgun oiled to prevent future rust.

value wise, its worth more with the original finish and some pitting than refinished.

What he said ^^^

what he said, you could replace the barrel

Value only matters if you intend to sell it. If you want to remove the pitting and reblue it, do it. I'd rather have a beautiful shotgun than one with pitting.