Dark Blue Black vs Hospitallier Black

How close are they?

ΔE 15.7, loose: different enough that you should pick on purpose. Hospitallier Black is a little darker, much more muted than Dark Blue Black.

Can I use Hospitallier Black instead of Dark Blue Black?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Hospitallier Black is a little darker, much more muted than Dark Blue Black.

Can I use Dark Blue Black instead of Hospitallier Black?

The same difference applies the other way round: Dark Blue Black 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.