ΔE 7.8, close: similar, with a visible difference side by side. Flat Black is a little lighter than Abaddon Black.
Usually, if you are not matching paint already on the model. Close match (ΔE 8). Flat Black is a little lighter than Abaddon Black.
The same difference applies the other way round: Abaddon Black is a little darker.
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 Abaddon Black on the colour wheel See Flat Black on the colour wheel
All matches: Abaddon Black equivalents · Flat Black equivalents