ΔE 13.3, fair: the same colour family, but not a straight swap. Blue Ink is a little darker, more saturated, warmer than Incubi Darkness. They are different kinds of paint: Citadel Incubi Darkness is an opaque base/layer paint and Reaper Blue Ink is an ink, glaze or transparent 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 13). Blue Ink is a little darker, more saturated, warmer than Incubi Darkness. They are different kinds of paint.
The same difference applies the other way round: Incubi Darkness is a little lighter, more muted, cooler.
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 Incubi Darkness on the colour wheel See Blue Ink on the colour wheel
All matches: Incubi Darkness equivalents · Blue Ink equivalents