When borax is heated with CoO on a platinum loop, blue coloured...

On heating, borax loses its water of crystallisation and decomposes to give sodium metaborate and boric anhydride

${Na}_{2}{B}_{4}{O}_{7} \cdot 10{H}_{2}O\overset{\Delta}{⟶}{Na}_{2}{B}_{4}{O}_{7} + 10{H}_{2}O$

${Na}_{2}{B}_{4}{O}_{7}\overset{\Delta}{⟶}2{NaBO}_{2}$ (sodium meta borate) $+ {B}_{2}{O}_{3}$

On treatment with metal salt, boric anhydride forms metaborate of the metal which gives different colours in oxidising and reducing flame. For example, in the case of copper sulphate, following reactions occur

${B}_{2}{O}_{3} + CoO \to Co{\left({BO}_{2}\right)}_{2}$

(cobalt \left(II\right) metaborate)
Blue Bead