ΔE 20.8, loose: different enough that you should pick on purpose. Obsidian Black is noticeably darker, much more muted than Motherlode.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Obsidian Black is noticeably darker, much more muted than Motherlode.
The same difference applies the other way round: Motherlode is noticeably lighter, much 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 Motherlode on the colour wheel See Obsidian Black on the colour wheel
All matches: Motherlode equivalents · Obsidian Black equivalents