Try overlaying a UV map and have a look at the model with the map in modelviewer. Google 'UV map grid' and import one of the images into PS as the top layer of your livery. I use the first one from the google results, useful for hard-to-find parts of skins.
If you just want to colour that marked part in black, just draw a filled rectangle, providing it won't overlap any other part of the texture, or use the the path tool and fill. I don't know about the A-10 textures, but if the cannon isn't part of the fuselage you'll have to paint that too and align it.