Dark Emerald vs Blackened Steel

How close are they?

ΔE 27.8, loose: different enough that you should pick on purpose. Blackened Steel is noticeably lighter, much more muted than Dark Emerald.

Can I use Blackened Steel instead of Dark Emerald?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Blackened Steel is noticeably lighter, much more muted than Dark Emerald.

Can I use Dark Emerald instead of Blackened Steel?

The same difference applies the other way round: Dark Emerald is noticeably darker, 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.