Yosef Karo, Translation:Shulchan Aruch — Orach Chaim 395…
“ When should he bless? When he gathers it from the residents of the alley or when he transfers possession to them. Rem"a: If they collected to make an eruv and forgot to bless on it, it is an eruv, as the blessing is not prohibitively necessary (Agur in the name of Shibbolei Leket, see the end of chapter 367) . ”
