ΔE 14.5, fair: the same colour family, but not a straight swap. Obsidian Black is much more muted than Grave Robber.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Obsidian Black is much more muted than Grave Robber.
The same difference applies the other way round: Grave Robber is much more saturated.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Grave Robber on the colour wheel See Obsidian Black on the colour wheel
All matches: Grave Robber equivalents · Obsidian Black equivalents