In case you weren't aware, when a boss dies and theres a unique in the chest, you get the Devourer aura effect around your character. This aura is also available for completing 100 hm runs, and it looks like black and cyan smoke. As a cosmetic aura its about the right size and colour to be seen but not obscuring, but as a loot aura to get you excited about opening the chest its just not enough, especially with the dark lighting in the sanctum.
Couple of ways I thought of that might make it easier to see:
make it absolutely massive
put it around the chest not the character
brighten up the cyan parts of the aura to really glow