Dwarf Shield vs Obsidian Black

How close are they?

ΔE 31.5, loose: different enough that you should pick on purpose. Obsidian Black is a little darker, much more muted than Dwarf Shield.

Can I use Obsidian Black instead of Dwarf Shield?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 31). Obsidian Black is a little darker, much more muted than Dwarf Shield.

Can I use Dwarf Shield instead of Obsidian Black?

The same difference applies the other way round: Dwarf Shield is a little lighter, 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.