We developed an Inverse Photometric method (IPM) to determine global physical parameters of RR Lyrae stars exclusively from multicolor light curves (S\'odor, Jurcsik & Szeidl, 2009, MNRAS, 394, 261). We showed that for good quality photometric observations of unmodulated RRab stars, the IPM gives similarly good results as direct Baade-Wesselink analyses do, but without the need for spectroscopic measurements. In the course of the development, we payed special attention to the applicability of the IPM for modulated RR Lyrae stars. Since there is no simultaneous spectroscopic radial velocity and photometric observations of any Blazhko star with good phase coverage both in pulsation and modulation, which would allow spectroscopic Baade-Wesselink analysis, the IPM is the only possibility today to study changes in the global physical parameters of Blazhko RR Lyrae stars during the modulation cycle. With the IPM, we have studied the extensive multicolor light curves of 4 Blazhko RRab stars we observed with the 24-inch telescope of the Konkoly Observatory during the past 5 years in the frame of the Konkoly Blazhko Survey. Small but unambiguous changes in the pulsation-averaged mean temperature, mean radius and mean luminosity have been detected in each star. Results on these Blazhko stars (SS Cnc, DM Cyg, RR Gem, and MW Lyr) are shown in this paper.