Just my 2 pence worth.
Many fish (especially Bichirs) will change their body colors and or patterns to blend in with their surroundings.
It looks to me like the fish in question is blind.
I have had many blind Bichirs that go black as it only sees black and is trying to blend in with what it thinks is it's surroundings.
It is (unfortunately) becoming quite common these days for breeders/sellers etc to purposely blind their fish to make more money as a "Black" ....
This is my knowledge and experience with Bichirs, but I'm pretty sure Arowana colors change due to their surroundings too. (Don't quote me)
Good luck with that Jar, it is unique!
* doh! Just realised some plonker has bumped this thread from like 5 years ago!