Based on the analysis of the established empirical relationship between the elastic constants and the melting temperatures for cubic pnictides of group IIIa elements, the melting temperatures of hypothetical cubic BSb and BBi have been predicted. A critical analysis of the melting data for the (super)hard cubic boron pnictides BX (X = N, P, As, Sb, Bi) has been performed, and it has been shown that their melting temperatures at ambient pressure, as well as their Vickers hardness, are a linear functions of the pnictogen Mendeleev number.