Box dye over ombré

[LexieLoos]

I've just had an ombré colour done on my hair, and I hate it!
The hairdresser did exactly what I asked, but I really don't like the colour. I'm used to being an all over golden blonde and now I have mousy roots with ashy ends, and I feel like Lady Di
My question is, if I put a blonde box colour all over, what's likely to happen to the darker roots?
Any help muchly appreciated!