i died min gas tank on my t-maxx black, it came out more tinted and you could still see the fuel, i just removed the cap to dye it. and wiped down the inside to get any RIT residue out, i left it in as long as the other parts i was dyeing at the same time. i dont think it will dye solid black because of the kind of plastic, like lexan wont dye.
only pic i have that you can see it tinted #5

here too, i lightened the pic just for visability