ΔE 12.9, fair: the same colour family, but not a straight swap. The Darkness is noticeably lighter, more saturated than Black.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). The Darkness is noticeably lighter, more saturated than Black.
The same difference applies the other way round: Black is noticeably 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.
See Black on the colour wheel See The Darkness on the colour wheel
All matches: Black equivalents · The Darkness equivalents