There is obviously some kind of scientific principle at play with Reaper and Moira. Since Blizzard has not quantified it for us I can call it the dark force, shadow, or pig lips if I want when I refer to it. They are obviously moving through some kind of pocket dimension of darkness when they teleport and both use it to sap life and heal themselves.
It isn't just a color scheme. The yellow energy that heals is biotic fields, the black is something that has not been quantified yet by Blizzard.