Grimdark Shadow vs Oblivion Black Wash

How close are they?

ΔE 26.8, loose: different enough that you should pick on purpose. Oblivion Black Wash is noticeably lighter, more muted than Grimdark Shadow.

Can I use Oblivion Black Wash instead of Grimdark Shadow?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Oblivion Black Wash is noticeably lighter, more muted than Grimdark Shadow.

Can I use Grimdark Shadow instead of Oblivion Black Wash?

The same difference applies the other way round: Grimdark Shadow is noticeably darker, 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.