ΔE 26.1, loose: different enough that you should pick on purpose. Smalt Hue is a little darker, more muted, warmer than Blue Flux.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Smalt Hue is a little darker, more muted, warmer than Blue Flux.
The same difference applies the other way round: Blue Flux is a little lighter, more saturated, 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 Flux on the colour wheel See Smalt Hue on the colour wheel
All matches: Blue Flux equivalents · Smalt Hue equivalents