ΔE 9.2, close: similar, with a visible difference side by side. Blackroot Grey is a little lighter than Hardened Carapace.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Blackroot Grey is a little lighter than Hardened Carapace.
The same difference applies the other way round: Hardened Carapace 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 Hardened Carapace on the colour wheel See Blackroot Grey on the colour wheel
All matches: Hardened Carapace equivalents · Blackroot Grey equivalents