“stood on opposite sides of the ark.” The Israelites were about to divide into two groups, one to stand in front of Mount Ebal and one to stand in front of Mount Gerizim, and here the ark divides the two groups.
“the foreigner and citizen alike.” It is interesting that this was a mixed group. The “citizens” were the Israelites, although the Hebrew word does not exactly mean “citizen.” That is why the English translations differ: “citizen,” “home-born,” “native,” “native-born.”
“in front of Mount Gerizim and half of them in front of Mount Ebal.” They were not way up the slopes of the mountains, but in front of them, fairly close together, which allowed the people to hear each other well.
“to bless.” The subject of the verb shifts from the people to whomever is doing the blessing, who is left out of the sentence. It might be Yahweh, the priests, or even Joshua (cp. Josh. 8:34).