I dont want to start any arguments or conflicts....leakdown is useless on a nitrous engine. you have to do compression test.

Block the carb open, take all the plugs out. Then compression test it, record the first pump then what it does after 5-6 pumps. for reference, my engine pumps 90 then goes to 130 after 6 pumps. (YES this is what i mean, nitrous cams that open the exh early give compression results like this)

This is also my routine maintenance, after every race i check the valve lash, the valve spring seat pressure, and compression test.

Leak down can fool you , compression test never lies.

Your "black" plugs could be from a lot of issues....if there is that much oil pumping the thing should be smoking like crazy...i would start with a compression test