Flesh Wash vs Black Wash

How close are they?

ΔE 27.5, loose: different enough that you should pick on purpose. Black Wash is noticeably darker, much more muted than Flesh Wash.

Can I use Black Wash instead of Flesh Wash?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Black Wash is noticeably darker, much more muted than Flesh Wash.

Can I use Flesh Wash instead of Black Wash?

The same difference applies the other way round: Flesh Wash 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.