The authentic Askenazi minhag is not to say barchu.
Are you referring to automatically saying barchu after every maariv, even if no one missed the first barchu – or are you referring to someone saying a makeup barchu only when someone missed the first barchu?
In EY, the Ashkenazim picked it up from the Sefardim.
Which Ashkenazim? Outside of EY Ashkenazim do it differently than in EY?