Cops have arrested a 17-year-old in connection with the killing of Brooklyn security guard Anthony Blair, who died Sunday on Atlantic Avenue. Alexander Singh, Blair's stepson and housemate, is in custody for the early morning neck stabbing of the 44-year-old Guyanese immigrant, reports the Daily News. According to a source the two men, who lived in Bed-Stuy, quarreled about Singh's mother before Blair was found dead near a Getty gas station.