Highlights on ombré now have an annoying line

Hi,
I tried doing a loreal ombré to my medium dark hair. I used a nice and easy root touch up in dark brown for the top also. So it went almost black on top and orange at the bottom  🙁 so then I decided to go to a hairdressers and get highlights so now I'm left with this ? (see photo.) Through the highlights there's a really dark annoying line where the ombré was and the bottom is a brassy block blonde. I actually like the highlights but can't stand the this dark line and the brassy bottom so have just been putting it up.  I don't want to go to the hairdressers and spend another fortune so I thought I could bleach the dark bits in my hair so it's all one colour then put a blonde colour over it. Would this work? Or have any other suggestions I'm open to anything right now  *gun*