Cr-Cl boracite single crystals (Cr3B7O13Cl) were studied at 10-300 K by optical and elec. characterizations under simultaneous visual control of the ferroelec. domain states. The temp. dependence of both the spontaneous birefringence Dns and the spontaneous polarization PS indicates clearly the 1st-order feature of both the .hivin.43m/.hivin.42m phase transition at T1 = 264 K and the .hivin.42m/mm2 one at T2 ~ 160 K. The optical indicatrix of the orthorhombic phase is biaxial pos. with the optical axes lying in the a-b plane and the index nb//PS//{001}cub. The important role played by the simultaneous optical domain control and the influence of the nature of electrodes on the measurements are also pointed out.