The Mishna Berurah writes that one should be careful to avoid making an unnecessary bracha. Rabbi Vosner (Shevet Halevi 10:45) writes that there are a number of gradations concerning this:
1- One who says Hashem’s name in vain, not in the context of a bracha, transgresses a Torah prohibition of לא תשא.
2- One who says a ברכה לבטלה, bracha in vain, according to the Rambam also transgresses the prohibition of לא תשא, but according to Tosfos this is only a Rabbinic prohibition.
3- One who says an unnecessary bracha has transgressed a Rabbinic prohibition.
Our custom today is not to make a separate bracha on the tallis kattan (in the morning) and to fulfill the Bracha on Tallis Kattan, when reiting the bracha on the tallis gadol. The Mishna Berurah (24) writes that one should have specific kavanna to fulfill the obligation of tallis kattan when making the bracha on the tallis gadol. The Aishel Avraham adds that one may (once in one's lifetime) make his intention known that whenever he makes the bracha on the tallis gadol, he is including the tallis kattan with that bracha.