ΔE 27.5, loose: different enough that you should pick on purpose. Macragge Blue is noticeably darker, much more saturated, warmer than Bismark's Grave.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Macragge Blue is noticeably darker, much more saturated, warmer than Bismark's Grave.
The same difference applies the other way round: Bismark's Grave is noticeably lighter, much more muted, 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 Bismark's Grave on the colour wheel See Macragge Blue on the colour wheel
All matches: Bismark's Grave equivalents · Macragge Blue equivalents