First off, please don’t think I’m trying to patronise!
I’m a little concerned by your methodology...
Direct dye must almost always be applied to a level 9/10 with the exception of red can be a level 6. ( true red is a level 7)
I’m more concerned that House Beauty applies blue to a level 6, that will fade out very ugly! The blue will cover now, but after a few washes; Brass + Blue equals yuck!
The way direct dye works means that the base colour greatly affects the colour outcome. Levels 6-8 are naturally way too brassy for blue, even navy/denim blue.
My advice would be to get your hair done by a professional alternative colourist. The blue in the pic is actually a custom mix, I’m guessing Pulp Riot or Pravana... I can tell by the ‘smoky’ effect.
Please don’t do this yourself, without the needed professional knowledge in fashion colours and blonding, it won’t look like the picture and it’ll honestly just be a waste of your time and resource.
Aha, I understand
The clearer you are with what you’d like to achieve and how you intend to execute the process, the easier it is for others to understand your process and help you along! Always include the whole story.
I agree that it may be a harsh journey...
But you can do it efficiently.
The best course of action here is to remove the level 4 colour with a colour remover. This will speed up the lifting process monumentally and allow you to then lift out the true base quickly without months of foiling.
Or you could do the easy method...
As she’s already dark, just get some extensions in there for a blue ombré.
I think that’d probably be my call.