Blackened Steel vs Obsidian Black

How close are they?

ΔE 6.8, close: similar, with a visible difference side by side. Obsidian Black is a little darker than Blackened Steel.

Can I use Obsidian Black instead of Blackened Steel?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Obsidian Black is a little darker than Blackened Steel.

Can I use Blackened Steel instead of Obsidian Black?

The same difference applies the other way round: Blackened Steel is a little 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.