What you're doing is fine.
I think black poop is infection. I've only seen it one other time and eventually that black turned into white mucasy stuff that was definitely infection.
Well in this case I would normally suggest treating with medicated pellets.. but since he won't eat you'll have to treat the water. Honestly, I suspect internal parasites but I don't really have any experience treating water for internal parasites. Another user here reported they used Jungle Parasite Clear successfully for this, but this is really really hard on the kidneys so proceed with caution. Otherwise you could try antibiotics which go internal like Kanaplex or Maracyn plus and hope it's the secondary infection that's causing him to not want to eat.
Just be aware that in his delicate condition both treating and not treating is risky.. so you'll have to use your best feeling but black poop may mean there is something to be treated and it may not just be the ornament.
Forgot about the magnet trick.. let us know..