ΔE 12.7, fair: the same colour family, but not a straight swap. Rusty Metal is noticeably darker than Blackened Steel.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Rusty Metal is noticeably darker than Blackened Steel.
The same difference applies the other way round: Blackened Steel is noticeably lighter.
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.
See Blackened Steel on the colour wheel See Rusty Metal on the colour wheel
All matches: Blackened Steel equivalents · Rusty Metal equivalents