(Credit: Zenith Richards / Met Opera) Terence Blanchard’s “Champion,” based on the true story of the troubled former middleweight boxing champion Emile Griffith, will make its highly anticipated Metropolitan Opera premiere. The opera, which opens on April 10,  will be performed nine times through May 13. The work, with a libretto by Michael Cristofer, will be conducted by Yannick Nézet-Séguin {…}