I had something similar happen just 2 weeks before Anelise was born. I wanted to go slightly darker as at the time I was blonde and I didn't want to have to bother too much with regrowth. So the hairdresser & I agreed on a colour that was perfect and I was to have some caramel & light blonde foils to blend everything in.
Anyway, the main colour they used came out close to being black. I was told it would fade and I would need to go back in 4 weeks to have it touched up. It hasn't faded. My regrowth is noticeable because it is lighter than the dark and it is horrific. I haven't had the time to go and sit in a chair for hrs on end to have it all done again. PITA.
I did complain when I was there but they told me it would fade and all would be fine. I guess looking back now they were taking advantage of the fact that I was due to have a bub around the time the 'fading' was meant to happen and would have other things on my mind apart from my hair. I will never go back to them again for colouring.

I hope you get yours corrected for free and it comes out perfect