I always used semi-permanent dyes (Manic Panic, Wild Color) when I dyed my hair blue because I never found any permanent ones. However I always (aside from once) rebleached my hair before dying it blue again or else it wasn't vibrant and stayed dark blue/greenish. I don't know if you dye you used also has lifting agents in it, if not then it won't show up (if I remember correctly you have naturally dark hair?) If you want to save your hair some damage you could try just bleaching your roots and then dying your whole head, but it wouldn't be as vibrant. That's my two cents. Post a picture!