In the Lassaigne test for nitrogen, the Prussian blue colour is due to the formation of

Nitrogen is converted to $\mathrm{NaCN}$ on sodium fusion. With $\mathrm{FeSO}{}_4$ this gives $\mathrm{Na}{}_4[\mathrm{Fe}(\mathrm{CN}){}_6]$, and the $\mathrm{Fe}{}^{3+}$ formed by air oxidation then gives ferric ferrocyanide, $\mathrm{Fe}{}_4[\mathrm{Fe}(\mathrm{CN}){}_6]{}_3$, which is Prussian blue.