Blue Steel vs Sapphire Blue

How close are they?

ΔE 36.6, loose: different enough that you should pick on purpose. Sapphire Blue is a little darker, much more saturated than Blue Steel.

Can I use Sapphire Blue instead of Blue Steel?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 37). Sapphire Blue is a little darker, much more saturated than Blue Steel.

Can I use Blue Steel instead of Sapphire Blue?

The same difference applies the other way round: Blue Steel is a little lighter, 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.