ΔE 28.1, loose: different enough that you should pick on purpose. Black Legion is noticeably darker, much more muted than Turnbull Turquoise.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Black Legion is noticeably darker, much more muted than Turnbull Turquoise.
The same difference applies the other way round: Turnbull Turquoise is noticeably lighter, much more saturated.
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 Turnbull Turquoise on the colour wheel See Black Legion on the colour wheel
All matches: Turnbull Turquoise equivalents · Black Legion equivalents