Assigning a value is tough, because gun values aren't commonly set for non-original finishes.
So, it comes down to what someone is willing to pay.
You could probably increase the value by stripping the black paint off the wood and refinishing it with a good finish.
I'm assuming the paint was applied to the metal to cover a worn or rusted blue finish, but you never know, it could have a decent blue job under the paint and someone painted it just to get a different look.