ΔE 10.0, fair: the same colour family, but not a straight swap. Midnight Blue is a little lighter, more saturated than Matt Black Primer. They are different kinds of paint: Army Painter Matt Black Primer is a technical paint or medium 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 Matt Black Primer. They are different kinds of paint.
The same difference applies the other way round: Matt Black Primer 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 Matt Black Primer on the colour wheel See Midnight Blue on the colour wheel
All matches: Matt Black Primer equivalents · Midnight Blue equivalents