Blackened Brown vs Dark Red

How close are they?

ΔE 7.4, close: similar, with a visible difference side by side. Dark Red is a little lighter, cooler than Blackened Brown.

Can I use Dark Red instead of Blackened Brown?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Dark Red is a little lighter, cooler than Blackened Brown.

Can I use Blackened Brown instead of Dark Red?

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