Despite the remarkable uniformity of this single expression, the Zohar on our parsha explains that the word "barukh" or "blessed" in this phrase has a radically different meaning depending on whether we are saying a blessing or a prayer.
The Zohar states that it is a profound insight "to know that all blessings on commandments, enjoyments and praises of this world come to pour out blessing from on high to the lower world. Unlike the blessings of prayer, which have their effect first in the direction from lower to upper, and only afterwards from the upper to the lower". (Zohar Ekev, III:271a.)
Here is one way of understanding this insight: The word "barukh", "blessed", expresses the fact that HaShem is the source of all berakhot - not like any earthly source of blessing and plenty which today gives forth and tomorrow may fail, but rather the Creator and Emanator of blessing itself. In this case the "passive" construction doesn't indicate something which is acted on but on the contrary something which is inherently possessed of a certain quality. (Similar to the word "chasid", meaning a pious person also a passive construction.)
When we apprehend the Divine aspect of some material experience, we "pour out blessing from on high to the lower world". By reciting a berakah, we testify and bring about that lighting Shabbat candles is not a mundane act of providing light at nighttime; it is an act which is infused with holiness because it is a mitzva. When we recite a blessing on food, we testify and bring about that we are not merely experiencing animal, material enjoyment; our souls are experiencing a special benefice from the Creator.
This is all well and good when the object of our blessing is before us, as it must be before we say any benediction of this type. Indeed, it is a grave transgression to say a vain blessing, one in which the object of blessing is not present as the blessing is recited.
But in our prayers, we specifically relate to what the world lacks from a physical point of view! We ask HaShem to send healing, rain, redemption, and so on. It is not possible to "pour out blessing from on high to the lower world" because the aspect of the lower world which the blessing relates to is presently lacking. In this case, we are first relating to HaShem as the Provider of material blessings, and secondly as the One Who infuses these blessings with sanctity.
Our prayers have their effect first in the direction from lower to upper, and only afterwards from the upper to the lower. During prayer, we need to elevate the world, to testify that the material needs of the world are of spiritual importance, so that His providence will provide them. At the same time we need to acknowledge that these sought-for blessings indeed have their source in G-d.
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