ΔE 25.3, loose: different enough that you should pick on purpose. Darkness Grey is noticeably darker, much more muted than Ahriman Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Darkness Grey is noticeably darker, much more muted than Ahriman Blue.
The same difference applies the other way round: Ahriman Blue is noticeably lighter, much 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 Ahriman Blue on the colour wheel See Darkness Grey on the colour wheel
All matches: Ahriman Blue equivalents · Darkness Grey equivalents