What do they do at a hair salon for a color correction?

My natural color is blonde which was then bleached lighter and then was dyed black with a semi permanent dye. What would they have to do and would it be safer for my hair to expand the process over a few weeks?What do they do at a hair salon for a color correction?
I have a friend that had a color correction, He went to the salon and they put some sort of stuff on his head that looke like shampoo and it took the color right out then he bleached it a few days later at the salon.


It didnt seem to be a serious process at all...

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