The NYPD’s highest-ranking black official abruptly quit Friday instead of taking a new post, a surprising move that caught city officials off guard at a time when the department is trying to mend relationships with minority communities.Chief Philip Banks III was to have been named first deputy commissioner, second in command at the nation’s largest police force. Commissioner William Bratton already had announced the change and Banks was to have been promoted Monday. Instead, he will retire. It wasn’t clear why Banks changed his mind, and Bratton cited only “personal and professional factors.” …