Hello,
First of all let me start by saying that my natural hair color is dark brown.. A year ago I decided to dye it blond but then just a few months ago I wanted to go black so I dyed it again..
The only problem is that the black keeps on fading and pieces of my blond keeps on showing up-not only that but it also gets lighter.. I dyed it twice already but it keeps on fading with every wash..
What can I do?
I was thinking to just dye my hair back to my natural color 2 see if that could help but I isn't sure.. Iam really tired of all the dying and just want my hair to rest but I don't want to look weird also.. I like the black but it keeps on coming off..
Ive gone to the salon but the same thing happends a few months later.. any suggestions?
Thank you:)

It appears your hair is porous. The hair can't hold onto the colour molecules because hair shaft is open/damaged.
First, use hair products formulated for coloured hair. Have weekly conditioning treatments, I personally love hot oil treatments with olive or coconut oil. You warm up the oil, and apply it to the hair, you can leave the roots out if you want to or just go easy on the roots. Pop on a plastic cap and leave on for 30mins or longer. Heat helps the hair to penetrate the treatments.

Another thing is, I would usually use a quazi/demi permanent on your hair because the colour molecules are different sized, big and small. But a permanent colours molecules are smaller and they could come out of the hair easily whilst bigger molecules from the quazi would stick into the hair better.

Hope that helps. :)