ΔE 14.6, fair: the same colour family, but not a straight swap. Obsidian Black is noticeably darker than Dark Aluminium.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 15). Obsidian Black is noticeably darker than Dark Aluminium.
The same difference applies the other way round: Dark Aluminium is noticeably lighter.
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 Dark Aluminium on the colour wheel See Obsidian Black on the colour wheel
All matches: Dark Aluminium equivalents · Obsidian Black equivalents