ΔE 6.9, close: similar, with a visible difference side by side. Ultramarine Blue is a little lighter, cooler than Matt Bluish Black. They are different kinds of paint: AMMO by Mig Matt Bluish Black is an ink, glaze or transparent paint and Army Painter Ultramarine Blue is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Ultramarine Blue is a little lighter, cooler than Matt Bluish Black. They are different kinds of paint.
The same difference applies the other way round: Matt Bluish Black is a little darker, warmer.
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 Bluish Black on the colour wheel See Ultramarine Blue on the colour wheel
All matches: Matt Bluish Black equivalents · Ultramarine Blue equivalents