I don't suppose deteriorating paint is impossible, but it does seem unlikely. More like either gasses backing up into the intake during the normal combustion process, or excess blowby gasses coming in from the PCV. But in the case of the PCV, that action takes place under the throttle blades and inside the manifold, so is also not as likely.
But it does not seem likely either, that it can be anything to do with the fuel tank or plumbing. Where do you have the two filters, and why? Do you have the stock sock-filter on the end of the sending unit in the new tank? Then one up near the carburetor presumably? Other one is inline somewhere between the tank and pump?
That layout is not out of line, with the additional filter, but it's one more than the factory had and is also not likely causing the black (soot?) issue.
However, if you ever start feeling the engine stumble under load, or start to run poorly when going uphill or at high speeds on the highway, it might be one or more of the filters starting to clog up.
What carburetor? Choke opening up all the way and relatively quickly?
Open element air filter?
How is the PCV system plumbed in, and how is the EVAP/charcoal canister plumbed in?
How fast does it turn black after cleaning?
And I guess the last question is, how does it run?
No matter what though, it does not sound to me as if it's related to tank venting.
Paul
