ΔE 11.8, fair: the same colour family, but not a straight swap. Obsidian Black is noticeably lighter, more muted than Blue Steel.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Obsidian Black is noticeably lighter, more muted than Blue Steel.
The same difference applies the other way round: Blue Steel is noticeably darker, 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 Blue Steel on the colour wheel See Obsidian Black on the colour wheel
All matches: Blue Steel equivalents · Obsidian Black equivalents