ΔE 16.2, loose: different enough that you should pick on purpose. Eldar Robe is much more saturated, warmer than Thousand Sons Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Eldar Robe is much more saturated, warmer than Thousand Sons Blue.
The same difference applies the other way round: Thousand Sons Blue is 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 Thousand Sons Blue on the colour wheel See Eldar Robe on the colour wheel
All matches: Thousand Sons Blue equivalents · Eldar Robe equivalents