When curator Denise Murrell talks about Édouard Manet, she races through the details of his life, sometimes abruptly changing course to parse less-discussed aspects of his art — often in disbelief that she’s among the few who have paid attention to these parts of him. After she has so convincingly retold his life story, you can’t help but see what she considers obvious: In his famous 1863 painting “Olympia,” Manet wanted the black maid to stand out.