ΔE 17.1, loose: different enough that you should pick on purpose. Blue Mountain is noticeably lighter, warmer than Incubi Darkness.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Blue Mountain is noticeably lighter, warmer than Incubi Darkness.
The same difference applies the other way round: Incubi Darkness is noticeably darker, cooler.
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 Incubi Darkness on the colour wheel See Blue Mountain on the colour wheel
All matches: Incubi Darkness equivalents · Blue Mountain equivalents