Black Templar vs Dark Green Blue

How close are they?

ΔE 25.4, loose: different enough that you should pick on purpose. Dark Green Blue is noticeably lighter, much more saturated than Black Templar.

Can I use Dark Green Blue instead of Black Templar?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Dark Green Blue is noticeably lighter, much more saturated than Black Templar.

Can I use Black Templar instead of Dark Green Blue?

The same difference applies the other way round: Black Templar 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.