ΔE 15.5, loose: different enough that you should pick on purpose. Flat Blue is a little darker, much more muted, warmer than Eldar Robe.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Flat Blue is a little darker, much more muted, warmer than Eldar Robe.
The same difference applies the other way round: Eldar Robe is a little lighter, much 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 Eldar Robe on the colour wheel See Flat Blue on the colour wheel
All matches: Eldar Robe equivalents · Flat Blue equivalents