Rough Iron vs Darth Blue

How close are they?

ΔE 40.3, loose: different enough that you should pick on purpose. Darth Blue is a little darker, much more saturated than Rough Iron.

Can I use Darth Blue instead of Rough Iron?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 40). Darth Blue is a little darker, much more saturated than Rough Iron.

Can I use Rough Iron instead of Darth Blue?

The same difference applies the other way round: Rough Iron is a little lighter, 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.