R. Ya'akov Kamenetsky, as quoted in Bi-Mehitzas Rabbenu Ha-Ga'on Rabbi Ya'akov Kamenetsky pp. 43-44:
It is customary to stand while reciting Birkhos Ha-Shachar [the blessings at the beginning of the morning prayer service]. Eventhough we do not find a source for this in Shulchan Aruch or Mishnah Berurah, and in the Gemara (Berakhos 60b) it seems we should do the opposite -- it says that when one awakes and sits on one's bed one recites the blessing "matir assurim", and so in Shulchan Aruch, Orach Chaim 46:1 -- nevertheless, it is a universal practice, and there is likely a source for it.And in the footnote of the editor:
See Siddur Beis Ya'akov of R. Ya'akov Emden (before the blessing on washing one's hands in the morning) where it says: A general rule is that all blessings that are praise and thanks are recited while standing.