This paper suggests a novel approach to the blind focusing of the electromagnetic field for microwave hyperthermia. The idea is to induce a contrast variation in the target and to exploit this variation for the synthesis of the excitations of the antenna array employed for the focusing, by performing a differential scattering measurement. In particular, the excitation vector is set as the right singular vector associated with the largest singular value of the differential scattering matrix, obtained as difference of two scattering matrixes measured by the antenna array itself before and after the contrast change. As a result, the approach is computationally effective and totally blind, not requiring any a priori knowledge of the electric and geometric features of the region hosting the target, as well as of its spatial position with respect to the antenna array.