You can tell that these black doors are factory painted/coated because the “lift here” stencils are present. If the tank crew had repainted them, they would have left off the stencil. Everyone knows that is the lifting point.
I agree, it looks like some kind of heat proof coating. In several photos posted above, you can see where it was painted over, then the heat burned the camo paint off, revealing the black underneath, like this one posted by @Mrclark7 :
Ken