All of our tefilos are directed to Hashem, not to any intermediary. To avoid any confusion, many strike from tefilah any references that would give the impression that we are addressing angels, e.g. as my son put it, “Machnisei rachamim [at the conclusion of slichos] is a nice tune, just leave out the words.” R’ Chaim Volozhiner even avoided saying “Barchuni l’Shalom” in Shalom Aleichem as it implies that we are asking angels for a blessing. Yet, in last week’s parsha we have a famous pasuk where Ya’akov Avinu does seem to call on an angel to bless his grandchildren -- “HaMalach hagoel osi ‘mol ra yevareich es ha’nearim…”
Rishonim (e.g. Seforno) explain that Ya’akov is not calling on an angel. The pasuk of “HaMalach hagoel…” is a continuation of the previous pasuk where Ya’akov calls on Hashem to bless Epraim and Menashe; he continues that call to Hashem and requests that Hashem send a “Malach hagoel” should his grandchildren not be worthy of Hashem’s direct intervention. The Ohr HaChaim alternatively suggests that Ya’akov simply refers to Hashem’s intervention in the world as a “malach” (see HaKsav v’Hakabalah on this point).
The first answer makes sense in context, but that intended of calling on Hashem to send a malach is surely distorted when we quote the bracha of “HaMalach hagoel…” independently of the previous pasuk as we do in Shema al ha’mita, when given as a bracha to kids getting aliyos on Simchas Torah, or when sung to a baby before a bris. Is the contextual meaning really implicit or intended in our quote (as R’ Shteinman suggests)? Or are we really just relying on the other explanation of the term “malach”? R’ Shteinman suggests another novel idea (based on Rishonim elsewhere): the term “malach” by definition means “messenger of Hashem.” Therefore, implicit in the term "malach" is the idea that Hashem is ultimately doing the blessing. This would seem to solve the issue with “Barchuni l’Shalom malachai hashalom” as well.