Dark Blue Black vs Mystic Blue

How close are they?

ΔE 25.9, loose: different enough that you should pick on purpose. Mystic Blue is noticeably lighter, much more saturated than Dark Blue Black.

Can I use Mystic Blue instead of Dark Blue Black?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Mystic Blue is noticeably lighter, much more saturated than Dark Blue Black.

Can I use Dark Blue Black instead of Mystic Blue?

The same difference applies the other way round: Dark Blue Black is noticeably darker, much more muted.

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.