Permanent hair color is washing out, hair stylists please answer!?

I went from a light/ medium blonde hair color to blue black. I used L'Oreal Preference box dye. It said on the box not recommended for people with very light hair. This was not the first time I've dyed my hair either but this is the first time I've dyed it this dark. The color looked wonderful for about the first week but it kept washing out every time I washed my hair. Now I'm left with a slight blue black color on top and ashy grey ends. The color on the deader parts of my hair didn't hold and the brassyness of my blonde is showing through. What is the reasoning behind this? I've been using shampoo and conditioner specifically for colored hair and the box dye was permanent. I'm baffled why the color didn't hold and it's only been 2 weeks or so. I'd like a hair stylist or someone who works in a salon to please answer this question. The only reason I can think of is some sort of chemical reaction and the "dead" hair on the ends not absorbing the color like it should.

Permanent hair color is washing out, hair stylists please answer!?

I am not a hair stylist but my best friend is. I had the same exact problem and she said you HAVE TO use shampoo and conditioner for color treated hair. Yep..I did that and it still faded out. She bought me some new stuff called "Kiwi". It locks the color in. I use the shampoo and conditioner and my color is staying in a heck of alot longer than it used too. Try it, its worth it.