ΔE 9.8, close: similar, with a visible difference side by side. Abaddon Black is a little darker, more muted than The Darkness.
Usually, if you are not matching paint already on the model. Close match (ΔE 10). Abaddon Black is a little darker, more muted than The Darkness.
The same difference applies the other way round: The Darkness is a little lighter, more saturated.
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 The Darkness on the colour wheel See Abaddon Black on the colour wheel
All matches: The Darkness equivalents · Abaddon Black equivalents