gemore 53 a hoio mehalech chutz lecrach and he saw a ohr if rov is akum he should`nt make a brocha vechulu. the mogein avrohom asks a question kol kovua kemechtze al mechtze (so even if rov is isroel how cold he make a brocho) ? and answers that he saw it from somebody on the street so kol deporush meribo porush a.k.d. the question is how come if its mectze al mechtze mevorchin since there is no kol deporush. (without the mogein avrohom you could ask really the same question just that you might argue that the gemore here might not be midin rov although it`s not mistavre)
It seems that the Pri Chadash (cited in the Gilyon ha'Shas of Rebbi Akiva Eiger) asks your question. He answers that here that is another Mi'ut of flames that was lit by gentiles from the flames of Jews on Motza'ei Shabbos. When this Mi'ut is combined with the Mach'tzeh, it results in a Rov of flames that rested on Shabbos and therefore one may recite the blessing.