How to dye bright, crazy coloured hair back to black?

Try and fade the turquoise as much as you can, then neutralise it by putting the opposite colour over it, so a red/orange if it is a blue green. If you can put a pic into negative that will show you the type of colour you need. Then put the Ebony over the top. I do it this way to stop it fading to a sludgy green. Adding more red to the Ebony would also work, but i find the first method more effective.

Ebony is red based now so you'd probably be fine putting it straight on top. I would suggest you still fade the color before dying though. Try anti dandruff shampoo or the bicarb method.

If you want more of a natural black, I would try to fade it down some first with fading treatments like these:
http://www.hairdyeforum.com/index.php?topic=1704.0
Then I would put an orange based red semi over it, then the ebony, which is more of a red/brown based black now.

If you want more of a blue black, I would still try to fade it a little so you don't get too much color buildup which could make the black fade out faster, then I would put Blue Velvet over it first and then a blue based black semi