Cobalt Blue vs Obsidian Black

How close are they?

ΔE 20.2, loose: different enough that you should pick on purpose. Obsidian Black is a little lighter, much more muted than Cobalt Blue.

Can I use Obsidian Black instead of Cobalt Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Obsidian Black is a little lighter, much more muted than Cobalt Blue.

Can I use Cobalt Blue instead of Obsidian Black?

The same difference applies the other way round: Cobalt Blue is a little 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.