Within the blackout zone of Lower Manhattan, residents who remained in the depopulated neighborhoods spoke Thursday of rising concerns over security. On the Lower East Side, would-be robbers posing as utility-repair workers had security officials in apartment buildings on high alert. While post-storm looting has occurred in other parts of New York City, there haven’t so far been major reports of crime in the darkened neighborhoods south of 34th Street. Henry Choi said three doormen — instead of one — stood at the entrance to his apartment building near Delancey and Forsyth, checking IDs of people going in and out....