ΔE 15.1, loose: different enough that you should pick on purpose. Obsidian Black is noticeably darker than Broadsword Silver.
Can I use Obsidian Black instead of Broadsword Silver?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Obsidian Black is noticeably darker than Broadsword Silver.
Can I use Broadsword Silver instead of Obsidian Black?
The same difference applies the other way round: Broadsword Silver 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.