Blonde to Black

You may just need another application or two of the demi since your hair is lighter to begin with. I went from a yellow to black using Wella's demi in 1N but it took 2 applications for it to stay black. I used that repeatedly over my length for a few years and it was still easy to remove with a color remover, I went right back to a gold in one go. Of course semi black are going to be less damaging but if you really want more of a permanent black I would stick with demis and just try reapplying it. You may need to do an inbetween color though like a warm brown so the color doesn't seem as transparent if your bleached base was very light + the demis still doesn't want to stay looking black.

So when you redid your roots you used it twice all the time? :O

No, it stuck to my natural dark ash blonde well. It's just that when I first went back to black it faded + looked a bit transparent after a few weeks, so I had to go back over the whole thing again and it was fine after that. When I redid my roots it covered just fine but a few months later you could tell they were a warmer looking black than the rest of my hair, like a soft black + not a jet black, so I'd overlap, but I'd go over my lengths anyway even if they didn't need it at all (which they never did after those first 2 application) because I thought I was going to have black hair forever and my hair always felt even better after dying it somehow, which is why I'd go over the whole thing. What is your natural color?

You can see what I mean here by the roots fading a bit:

I put a pic of my natural colour back when I was 15 xD
I think it might actually be a little darker by now, haven't seen it for a few years now.
Imma try putting another demi on then... they just always seem to make my hair feel terribly dry afterwards, no clue why. Too bad that the demi you used is soo expensive to get here.
I think since my hair is relatively short atm, my roots tend to stick out more when they're different.