Charles E. Bolton, as he was known in San Francisco, was a man about town. In the summer and fall he would leave the City to inspect his mining interests in the goldfields, or so he said. During the winter and spring he lived a life of leisure, dining in fine restaurants and strolling the streets, always well-dressed, always courteous.
When he was released from San Quentin in 1888, The Wasp, a satirical San Francisco magazine founded by Ambrose Bierce, published this cartoon—

Black Bart is dressed as an elegant Parisian gentleman, professing his love to a lady. Wells Fargo, the dainty lady, looks askance at his advances.
Here is how The Wasp described the cartoon in their January 28, 1888 issue.
I’m not sure why the artist chose to dress Bart as he did, but the style reflects Bolton’s reputation as a “man about town,” what the French would call a “boulevardier.” A boulevardier was a man of fashion, who was seen in the the best places around the city.
No wonder Black Bart caught the public’s fancy; he was a different sort of stagecoach robber — he was always news.