The Bracha and the Establishment of the Eruv
The bracha and declaration accompany the communal joining. They express the mitzvah being performed; they do not create walls or settle the status of the streets.
The bracha accompanies the act of joining
The bracha is made before the act that completes the eruv chatzeiros or shitufei mevo'os. It recognizes the mitzvah enacted by Chazal and the communal permission created through the shared food.
The accompanying declaration states that through this eruv the residents may carry between the joined domains and through the enclosed area in the manner halacha permits.
The declaration rests on an existing enclosure
The words do not define the city boundary. The Rav must already have determined that the area is enclosed appropriately and that the food and acquisition meet the requirements.
In a municipal eruv, the bracha is therefore one step near the end of a much larger process. It cannot settle reshus harabim, repair a gap, or supply missing sechiras reshus.
When the communal eruv is renewed
If the food is replaced or a new communal acquisition is made, the question of a new bracha follows the ordinary halachos of renewing the mitzvah.
The supervising Rabbonim keep the communal joining under review just as the physical route is inspected, although the two systems address different needs.