The Talmud specifies Nisan, but in the southern hemisphere there are no blossoms in Nissan, so it is logically inferred (as poskim indicate) that one could say the bracha in the correct month, which is Elul or Tishrei……So we must wonder why this kind of halachic adaptation is only allowed in certain circumstances but not in others. For instance, the concerns about possibly having tsaar during Sukkos would be automatic in colder countries, in which case people should unquestionably be considered EXEMPT automatically from Sukkah for the obvious reasons including the discomfort of sleeping, making the sukkah itself not fitting for the mitzvah (per the mitzvah requiring eating AND sleeping).
There is a difference between the bracha on the blossoms and succos. The reason for the bracha on the blossoms is not inherently because it is nissan, but because practically it is when the trees blossom. (See MIshna Berura O:CH 227-1)Therefore in countries where this is not applicable, and the trees actually blossom at a different time, then the person is benefiting from seeing the blossoms in a different month and therefore he should make the bracha then. Succos on the other hand is to celebrated specifically on the 15th of Tishrei, therefore it will be the same date any place on earth.