i don't know how to begin!

You could do a colour remover like colour B4 and others to get rid of the black dye in the pieces you want blue. Colour removers don't damage, they only dry a bit so use conditioner after. It's definitely worth it to try that before you resort to using anything with peroxide.

ow i'm sorry i didn't mention this but i live in the netherlands and as far as i know we don't have b4 or color remover.
it is a verry good suggestion because i saw the results of various persons on this forum but i don't think i can buy that here :'( :-[

You can buy it online.

I've heard Kruidvat also sells some sort of colour remover without bleach. I don't know what it looks like, or what it's called exactly, but if you don't want to buy online, this might be an option. 🙂

thank you very much! i will search for it at the kruidvat tomorrow ^^ and if that doesn't work i can also buy it online but that takes so loooooong xD but did anyone ever try what i suggested with the purple?and how did it work out?

It depends on how light you're able to bleach it to.

If you're not able to get a color remover and don't want to wait or pay the shipping and whatnot to order one, I would try a couple vitamin c treatments first. I've seen them work pretty well on freshly dyed dark brown, a few treatments bringing it up to a light brown/dark blonde on hair that had previously been pale yellow. It's worth a try at least 🙂

If you can't get it past a ginger shade then I would go for more of a warm dark purple first. Then when that fades + your hair has had time to rest you can try to lighten it further so you can get blue over it. If you use a strong and dark enough blue it will go over yellow just fine, but you should add some purple to it as well or magenta to help it from going green when it fades