In Angier's book on Blueing & Browning, page 18, he mentions that any carbonate in the boiling bath will 'partly or entirely convert the carefully formed black oxide into fox-red carbonate: a most unwelcome accident that is hard to repair even with a coarse scratch brush.'
I suggest that your boiling water had become corrupted with carbonate (hard water?) and this caused your problem rather than any problem with the temp of the bath.
When blacking barrels, I rarely bother to push the tank temperature to a full rolling boil, I find that the black oxide forms very happily at anything over 92 degrees. Having the tank right up to temp doesn't even seem to speed the process.
I must admit to no experience with blacking Damascus and if this was what you were doing, there may be a different issue.
