ΔE 13.7, fair: the same colour family, but not a straight swap. Flat Black is noticeably darker, more muted than The Darkness.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Flat Black is noticeably darker, more muted than The Darkness.
The same difference applies the other way round: The Darkness is noticeably lighter, more saturated.
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 The Darkness on the colour wheel See Flat Black on the colour wheel
All matches: The Darkness equivalents · Flat Black equivalents