ΔE 6.6, close: similar, with a visible difference side by side. Obsidian Black is a little darker than Mythril Blade.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Obsidian Black is a little darker than Mythril Blade.
The same difference applies the other way round: Mythril Blade 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 Mythril Blade on the colour wheel See Obsidian Black on the colour wheel
All matches: Mythril Blade equivalents · Obsidian Black equivalents