ΔE 21.2, loose: different enough that you should pick on purpose. Obsidian Black is noticeably darker than Blackened Steel.
Can I use Obsidian Black instead of Blackened Steel?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Obsidian Black is noticeably darker than Blackened Steel.
Can I use Blackened Steel instead of Obsidian Black?
The same difference applies the other way round: Blackened Steel 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.