ΔE 16.2, loose: different enough that you should pick on purpose. Black Wash is noticeably darker than Soulblight Grey.
Can I use Black Wash instead of Soulblight Grey?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Black Wash is noticeably darker than Soulblight Grey.
Can I use Soulblight Grey instead of Black Wash?
The same difference applies the other way round: Soulblight Grey is noticeably 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.