When borax is heated with CoO on a platinum loop, blue coloured bead formed islargely due to : 

Borax $\left({\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7\cdot 10{\mathrm{H}}_2\mathrm{O}\right)$ on heating gets fused and loses water of crystallisation. It swells up into a fluffy white porousmass which then melts into a colourless liquid and forms a clear transparent glass bead consisting of boric anhydride andsodium metaborate. 
${\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7.10{\mathrm{H}}_2\mathrm{O}\stackrel{\mathrm{\Delta }}{\to }{\mathrm{Na}}_2{\mathrm{B}}_4{\mathrm{O}}_7\stackrel{\mathrm{\Delta }}{\to }2{\mathrm{NaBO}}_2+\underset{\mathrm{B}}{ }{\mathrm{Br}}_2{\mathrm{O}}_3$
Boric anhydride is non-volatile. When it is reacted with a coloured metallic salt, a characteristic coloured bead of metalmetaborate is formed. Thus, when reacted to cobalt oxide, forms blue coloured $\mathrm{Co}{\left({\mathrm{BO}}_2\right)}_2$ bead.
${\mathrm{B}}_2{\mathrm{O}}_3+\mathrm{CoO}\to \mathrm{Co}{\left({\mathrm{BO}}_2\right)}_2$
           Cobalt metaborate (blue bead)