you go to drustvar and watch the witch magic there.
that is death magic, and it's blue and black. elune's dark moon magic is the same colors.
now, there is a thing to that... we don't know the extent to which they're using void with their death magic. they got their evil powers from the emerald nightmare, which we know is a mix of death and void, because it was made by yogg-saron using death and void magic.
when we see bwonsamdi use death magic, it's a teal greenish color. his is probably what pure death magic looks like.