ΔE 18.2, loose: different enough that you should pick on purpose. Mahogany is noticeably lighter, more saturated than Black Legion.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Mahogany is noticeably lighter, more saturated than Black Legion.
The same difference applies the other way round: Black Legion is noticeably darker, more muted.
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 Black Legion on the colour wheel See Mahogany on the colour wheel
All matches: Black Legion equivalents · Mahogany equivalents