ΔE 18.5, loose: different enough that you should pick on purpose. Ultramarine Blue is a little darker, much more saturated, warmer than Blue Steel.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Ultramarine Blue is a little darker, much more saturated, warmer than Blue Steel.
The same difference applies the other way round: Blue Steel is a little lighter, much more muted, cooler.
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.
See Blue Steel on the colour wheel See Ultramarine Blue on the colour wheel
All matches: Blue Steel equivalents · Ultramarine Blue equivalents