Some time last year I blocked black demons as a slayer task and then maybe a month later removed black demons from the block list. Ever since then I have not been able to get a single black demon task from either Steve or Duradel. I'm roughly around 70-80 slayer tasks dry of black demons.
Highly unlikely that it would be a bug though, you just gotta keep trying
Nieve/Steve has the highest probability of assigning you a black demon task, and as such I'd reckon you getting your tasks assigned from them to increase your likelihood of getting a black demon taskYou cannot reason people out of positions they never reasoned themselves into