ΔE 29.6, loose: different enough that you should pick on purpose. Flat Red is noticeably lighter, much more saturated, warmer than Blood Thorn.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 30). Flat Red is noticeably lighter, much more saturated, warmer than Blood Thorn.
The same difference applies the other way round: Blood Thorn is noticeably darker, 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 Blood Thorn on the colour wheel See Flat Red on the colour wheel
All matches: Blood Thorn equivalents · Flat Red equivalents