Metal surface composition due to the manufacturing process, possibly the barrel was made to size and never turned down. I bought an LW barrel for a Pro Target and this was the correct size that I needed it to be, and the blueing never took well, only got as far as that reddish violet you have there (it was blued twice...)
But, where the barrel was machined, perfect black...
Sadly no "fix" for you but then you don't want one so that's lucky
**WANTED**: WEBLEY PATRIOT MUZZLE END; Any Diana/Original mod.50 parts, especially OPEN SIGHTS