Why did Moshe enter the Ohel Mo'ed with Aharon at this point?
Rashi #1: In order to teach him how to bring the Ketores. 1
Rashi #2, Rashbam and Targum Yonasan: To Daven for the Heavenly fire (which is synonymous with the Shechinah) to descend to burn the Korbanos. 2
See Sifsei Chachamim.
When Aharon saw that, despite all the Korbanos he had brought, the Shechinah had not appeared, he became despondent, and convinced that Hashem was angry with him for his role in the Golden Calf, he accused his brother of embarrassing him. So Moshe went with him to the Ohel Mo'ed to Daven, and the Shechinah came down in the form of the Heavenly fire.
What B'rachah did Moshe and Aharon now confer upon the people, which resulted in the Shechinah descending?