The Darkness vs Blue Black

How close are they?

ΔE 7.7, close: similar, with a visible difference side by side. Blue Black is a little lighter, more saturated than The Darkness.

Can I use Blue Black instead of The Darkness?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Blue Black is a little lighter, more saturated than The Darkness.

Can I use The Darkness instead of Blue Black?

The same difference applies the other way round: The Darkness is a little darker, 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.