ΔE 24.2, loose: different enough that you should pick on purpose. Blue Mountain is noticeably lighter, much more muted, cooler than Naggaroth Night.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Blue Mountain is noticeably lighter, much more muted, cooler than Naggaroth Night.
The same difference applies the other way round: Naggaroth Night is noticeably darker, much more saturated, warmer.
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 Naggaroth Night on the colour wheel See Blue Mountain on the colour wheel
All matches: Naggaroth Night equivalents · Blue Mountain equivalents