Soulstone Blue vs Black Iron Oxide

How close are they?

ΔE 55.5, loose: different enough that you should pick on purpose. Black Iron Oxide is noticeably darker, much more muted than Soulstone Blue.

Can I use Black Iron Oxide instead of Soulstone Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 56). Black Iron Oxide is noticeably darker, much more muted than Soulstone Blue.

Can I use Soulstone Blue instead of Black Iron Oxide?

The same difference applies the other way round: Soulstone Blue 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.