Here is a little blurb on the subject;
http://www.panzerworld.net/blackmuzzles.html
On my recent trip to Germany, I had a chance to visit the Wehrtechnische Museum in Koblenz and the Deutsche Panzermuseum in Munster. One of the things I looked for was this very subject. Unfortunately, since most of the vehicles and guns are restored, any evidence of discoloration is long gone. However, there were some guns that were left in their original paint. There was no blackening evident on these weapons.
What was interesting though, was the belief in muzzle blackening, as some of the tanks actually were spray painted black on the muzzles! Here is an example, the lovely "Ute" at Koblenz;
This leichte I.G. 18 is in it's original paint. The muzzle doesn't look any more weathered than the rest of the piece;