ΔE 7.5, close: similar, with a visible difference side by side. Burnt Umber is a little darker than Blackroot Grey.
Usually, if you are not matching paint already on the model. Close match (ΔE 8). Burnt Umber is a little darker than Blackroot Grey.
The same difference applies the other way round: Blackroot Grey is a little lighter.
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 Blackroot Grey on the colour wheel See Burnt Umber on the colour wheel
All matches: Blackroot Grey equivalents · Burnt Umber equivalents