Steven Tyler made a surprise appearance at the Black Crowes’ May 15 concert London, delivering his first performance since suffering serious vocal cord damage last year. After rocking through their 16-song set, the Crowes returned to the stage for an encore. The band had just launched into Aerosmith’s classic song “Mama Kin” when Tyler walked onstage unannounced. It didn’t take long for the audience to recognize the famous frontman, as a loud cheer from the crowd soon erupted. Black Crowes vocalist Chris Robinson started the song off, handling the opening lines before Tyler grabbed the microph…