ΔE 26.7, loose: different enough that you should pick on purpose. Cygnar Blue Base is noticeably lighter, much more saturated, cooler than Naggaroth Night.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Cygnar Blue Base is noticeably lighter, much more saturated, cooler than Naggaroth Night.
The same difference applies the other way round: Naggaroth Night is noticeably darker, much more muted, 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 Cygnar Blue Base on the colour wheel
All matches: Naggaroth Night equivalents · Cygnar Blue Base equivalents