Getting Hair Back to Natural Color?

Now I've dyed my hair since I was a young girl. I'm naturally a blonde who has dyed her hair darker colors like browns and blacks my whole life, probably since the age of 4 because my mother had found out it was a remedy for head lice or such. But now I've been wanting my hair to go back to blonde. Of course I know I can exactly get it back to the color it was and I don't want it to be that blonde. I just want a shade of blonde and every time I dye my hair my roots come in black? Why is this?