General Motors Co said on Wednesday its North American labor chief retired last week, as the No. 1 U.S. automaker prepares to open negotiations with the union representing its U.S. hourly work force. GM said Rex Blackwell, its 60-year-old North America labor relations vice president, retired on June 1. In April, he had been promoted to lead negotiator for the coming talks over a new contract with the United Auto Workers union.