I'd disagree with that. Blacking, or bluing, is there as a controlled rusting to prevent undesirable rusting. If the metal has worn bare such that there is a risk of even a light rusting taking hold it needs redoing.
The same as paintwork on a house soffit or the paintwork on metal railings if it degrades such that what is underneath then is exposed to the the bare wood or the bare metal it needs redoing or renewing if it cannot be "touched up". My late father's gun made in 1919 is now wearing its second reblack on the barrels, guard, top lever and triggers. So blacked once when new in 1919, done again for a first reblack in the late 1960s or early 1970s and then done (the second reblack) in the late 1990s or early 2000s.
All tools that have served you well deserve also being served well by you. And when done properly...which may mean that any engraving needs to be picked up during the process so don't skimp on the overall costing...it looks correct.