Why did the Metropolitan Museum remove the John Wilson banner that last autumn had been gracing its façade? That arresting image was made from a tiny portrait of the artist’s brother, expanded to monumental public proportions, that announced the Met’s “Witnessing Humanity” exhibit, a Wilson retrospective that continues through February 8. In the picture, the brother’s brow is steadfast, his gaze grave and alert, mouth and chin resolutely composed; perhaps no Black face has ever so effectively stared down the self-regard of Manhattan’s Museum Mile. Wilson, who died in 2015, had made the portrait in 1942 as a 20-year-old art student; “In my youth,” he once said, “the Black man was an invisible American.” For some weeks along the fanciest part of Fifth Avenue, this had not been the case.