ΔE 45.6, loose: different enough that you should pick on purpose. Violet Wash is much more saturated than Oblivion Black Wash.
Can I use Violet Wash instead of Oblivion Black Wash?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 46). Violet Wash is much more saturated than Oblivion Black Wash.
Can I use Oblivion Black Wash instead of Violet Wash?
The same difference applies the other way round: Oblivion Black Wash is much more muted.
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.