Blonde to black but fading...

Hi! New to the forums here, but hoping you guys can help a newbie!
Over the years i have changed my hair color about a million times, from dark to red to blonde to hot pink, you name it. But always went to the salon if it involved bleach. Dying it darker i always do on my own from a box kit from walmart.
That said, I have had two toned hair for nearly a year now, been bleached nearly platinum, with a layer of my own naturally dark hair underneath. I understand i am basically a level 2 with how dark my natural color is, so this tells you the damage that was done to get it to lift, not to mention we stripped black dye out of it to bleach it blonde. It was a long day of double processing.
I'm recently unemployed, can't afford to keep up with the blonde and salon visits. Decided to dye it all jet black and i can keep that up from home to keep the grays out very inexpensively compared to a salon visit.
Before you say I shoulda gone to a salon to have it dyed back, my small salon uses product from Sally anyway and would've done the same thing i did.
At the suggestion of the girl at Sally's and a friend that does hair, i used the brand Colorful, Neutral Protein Filler...followed directions exactly...Then used the bottle of Jet Black Ion Dye, which is liquid form, with 2 oz of level 10 developer as I was told. It took great in most places and looks fantastic! But some areas there is apparently too much damage to hold color still and it's a deep brown after i washed the dye out and a bit more faded still after I washed this morning, a couple days later. It doesn't look awful, but I want it all jet glossy black.
Can i fix these areas and if so, how? Re-dye with filler? Re-dye without filler? Or is it a lost cause on the damaged area? I'm trying to let it grow, so I don't want to cut the damage out.
I thought the filler would keep it from washing out, but it has faded a bit on the more severely damaged areas each wash, and i'm afraid will continue to do so.
Brand names and directions would be greatly appreciated! Thanks for any help!!!
And Merry Christmas! :)