Q. Why when we say a brocho we begin by speaking directly to Hashem, as in Baruch Ata (blessed be You) and when we end the bracha we speak of Hashem in third person, as in Bore Pri Haetz (who created the fruit of the tree)?

A. The better-known reason is given by the Rashbo (Teshuvos 5: 52) who explains that there are two fundamental principles on which all of our beliefs rest. One is the truth of the existence and beingness of Hashem. That knowledge has to be so crystal clear in our minds, as if we would be standing and seeing Hashem in front of us and talking directly to Him. The second basic truth is that no creation of His, will ever be able to fully understand what He really is. His essence will always be for us, the fruits of His creation, hidden in mystery (nistar). Therefore the blessings contain the speech forms that represent those two fundamental beliefs.
Oitzros Habrocho (p. 167) adds another perspective. When one addresses Hashem directly, one should begin to realize to whom he is really speaking. Thus he should become totally overwhelmed with plain fright, deep felt awe, and respect. Those profound emotions would force a response to hide and seek refuge. Therefore we end the brocho in a third person grammatical form.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a