Hi Mark,
Thank you so much for starting that highly interesting topic!
The only real examples of painted/stained iron surfaces on early (15th-17th) firearms to prevent them from rusting I can give are minium/read lead painted barrels and locks. Blueing, on the other hand, was almost always achieved by the heat of the smith's fire and stopped at exactly the right color/temperature in those times.
What I have often seen in untouched arsenal situations are barrels painted black for the same conservatory reasons but only in the 18th century and later - which means well after their original time of use.
For today I give some samples from a minium painted barrel of ca. 1460-70 preserved in Schloss Ambras, Austria, and from Schloss Frondsberg, Styria/ Austria, ca. 1540 - the later partially sold by Tom Del Mar, 15 Dec 2004; the barrels of the latter were missing and later replaced by primitive dummies.
More to follow.
Best, my friend,
Michael
P.S. Please see also my previous thread on the world's oldest known existing hand firearm, now preserved in my collection!
http://www.vikingsword.com/vb/showthread.php?t=7077