1) Even though ordinarily a Beracha is made before the Mitzvah, when washing one’s hands one makes the Berachah after the washing, as there are times when the hands will be unclean before the washing. Since the drying of the hands is also a part of the Mitzvah, when one makes the Berachah before drying one’s hands it is considered as if one made the Berachah before completing the Mitzvah. (Shulchan Aruch and Mishne Berura 158)
2) One should be extremely careful to make the Berachah before drying one’s hands and not to leave it until afterwards. If, however, he forgot to make the berachah until after he dried his hands, he may make the Berachah then. (ibid)
