Terrestrial Titan vs Blue Black

How close are they?

ΔE 8.3, close: similar, with a visible difference side by side. Blue Black is a little darker, cooler than Terrestrial Titan.

Can I use Blue Black instead of Terrestrial Titan?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Blue Black is a little darker, cooler than Terrestrial Titan.

Can I use Terrestrial Titan instead of Blue Black?

The same difference applies the other way round: Terrestrial Titan is a little lighter, warmer.

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.