An aqueous solution of ${\text{CoCl}}_2$ on addition of excess of concentrated $\text{HCl}$ turns blue due to formation of:

Aqueous ${\text{CoCl}}_2$ exists as the pink octahedral complex $[\text{Co}({\text{H}}_2\text{O}{)}_6{]}^{2+}$. Upon adding concentrated $\text{HCl}$ (excess chloride ions), a displacement reaction occurs to form the blue tetrahedral complex $[{\text{CoCl}}_4{]}^{2-}$.
$$[\text{Co}({\text{H}}_2\text{O}{)}_6{]}^{2+}+4{\text{Cl}}^{-}\rightleftharpoons [{\text{CoCl}}_4{]}^{2-}+6{\text{H}}_2\text{O}$$