Welcome aboard! That's a tough one. The body of the LP53 has a kind of crackle-finish paint on it, and Walther say the barrel is anodised ... There must be something that binds to aluminium alloy and turns it black ... Can only think it might be an idea to ring up a company that makes scope mounts like Sports Match and see if they can help. They make things out of aluminium alloy and anodise them or black them some how, so they should know.
I LOVE the LP53 but it is a bit of a collectors item now (i.e. expensive!) so I have never owned one myself. Quality piece of kit it is too.
This company popped out of Google, but it may be that they don't do small jobs for individuals. Worth giving them a call anyway.
http://www.tecan.co.uk/UK/_03Metalfinishing.html