ΔE 4.4, very close: most people won’t see a difference side by side. Outlining Black is a little darker than Black Legion. They are different kinds of paint: Citadel Black Legion is a one-coat contrast-style paint and Nocturna Models Outlining Black is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Yes, for most jobs. Very close match (ΔE 4). Outlining Black is a little darker than Black Legion. They are different kinds of paint.
The same difference applies the other way round: Black Legion is a little lighter.
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 Outlining Black on the colour wheel
All matches: Black Legion equivalents · Outlining Black equivalents