ΔE 10.0, fair: the same colour family, but not a straight swap. Midnight Blue is a little lighter, more saturated than Black Legion. They are different kinds of paint: Citadel Black Legion is a one-coat contrast-style paint and Reaper Midnight Blue is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). Midnight Blue is a little lighter, more saturated than Black Legion. They are different kinds of paint.
The same difference applies the other way round: Black Legion is a little 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 Midnight Blue on the colour wheel
All matches: Black Legion equivalents · Midnight Blue equivalents