The Talmud establishes only very generally that Jews should prepare themselves mentally for prayer and that daily recitation of certain psalms is an ideal for prayer for which one should strive. However, in the earliest extant geonic treatises on prayer, the pesukei d’zimra, complete with a framework of blessings, are an established part of the morning service. Halakhot Gedolot, from the late ninth century, records an earlier ruling that the latecomer trying to catch up should abbreviate the psalms recited but not the blessings themselves, which clearly are already more essential liturgically than the psalms they came to frame. Natronai Gaon’s famous responsum detailing the hundred blessings one is required to recite daily simply lists barukh she-amar, assuming that his readers will know it. Although no geonic traditions attempted to hide the post-talmudic nature of the opening blessing particularly, none made excuses for its existence either. Most Rishonim did not question the legitimacy of this blessing and referred to its origins only vaguely as an enactment of the rabbis. (pp. 48-49; see also nn. 34-37 on pp. 49-50 on this selection)& the following:
Until the fourteenth century, most rabbinic discussions of its origins assumed the existence of some vague rabbinic enactment, the exact nature of which was of no great concern. The Tur mentions in passing that the blessing appears in Sefer Heikhalot. R. David ben Yehudah Hasid, in his influential kabbalistic prayer commentary, Or Zarua, states that barukh she-amar was not established by the men of the Great Assembly. Instead, scholars and men of wisdom received it directly from “the tradition of the covenant” when a note fell from heaven. R. David was no doubt rebutting a theory suggested by some unidentified group or person in his world. Obviously, it was not a mainstream theory, judging from the literature of the period. However, in the sixteenth century, R. Meir ibn Gabbai, in his commentary Sefer Tolaat Yaakov, miscites the Or Zarua, stating that it ascribes barukh she-amar specifically to the men of the Great Assembly, who established the prayer on the basis of a note that fell from heaven. This clever ascription removed all doubt about the legitimate origins of barukh she-amar by placing its provenance in the same mythical pre-talmudic antiquity as the amidah, the paradigmatic Jewish prayer. This explanation eventually became standard in noncritical prayer commentaries.
The impetus to generate and accept such “origins” for this single prayer must have come from the continuing need to justify its recitation. Before Ibn Gabbai’s solution was widely accepted, Hizkiah da Silva, in his Peri Hadash, a late seventeenth-century Sefardi commentary on the Shulhan Arukh, challenged the geonic right to establish new blessings, basing his argument on the Rosh’s objections to the priest’s blessing at the pidyon haben. Da Silva was either unaware of, or, more likely, unconvinced by the eighteenth-century commentators, the Peri Megadim and the Beer Heitev. The Beer Heitev, like his Sefardi contemporary, the Hida, also points to the Tur’s identification of the source of the blessing in the Sefer Heikhalot as proof of its pre-talmudic origins. (pp. 100-101; see also nn. 244-250 on pp. 100-101 thereupon)