What I have managed is to black-up all the parts I want to blacken on one of my tanks just so I can make sure it's working as an overall scheme. I've used the same scheme on my other Tau models so I'm not in much doubt, but there is a lot more to blacken on the tanks than the other models so the balance of dark-to-light is skewed by comparison.
I think it's still working though.
More Tau-related stuff:
Tau family album: