Hi Guy's,
For what it's worth I have seen quite a few Lewis guns and I have never seen one with any black paint of any kind on it.
Metal parts blued or blacked (chemically) like any other gun. Some Drum magazines SEEM to be in white, bare metal but that may just be excessive usage---I firmly believe most would be chemically treated. SOME barrels (the stripped ones_ are bright metal, but some I have seen are Blued.
The ONLY part I have seen regularly (I would almost say invariably) painted is the cooling jacket, and they are mostly Green but some were obviously, as can be seen on photos., painted a lighter colour----in the same way some Vickers were.
Cheers,
Dave.

EDIT--I should add some jackets appear to be blued/blacked also...