According to the information I can find online, Thomas Stewart was born in Kalamazoo, Michigan in 1823. He was one of the first African-Americans to be granted a patent. His first major invention was a station and street indicator that he patented in 1883. In 1887 he invented a metal bending machine used in the steel industry. Next, he invented his mop in 1893. It included two major innovations, a replaceable head and a spring-loaded clamp that let you wring out the mop without actually having to reach over and squeeze the mop head.
Three sites which have some information about these inventions are:
For additional details about Stewart, you might want to look at two books which should be available at your university library or via interlibrary loan:
W. Sherman Rogers. The African American Entrepreneur: Then and Now.
John Sibley Butler. Entrepreneurship and Self-Help among Black Americans
Another possible source would be nineteenth-century newspapers from Kalamazoo. If you live in the area, you might visit the WMU Archives and Regional History Collections, which should have the most extensive collection of primary texts:
I found the following Internet site with information about Thomas Stewart:
I researched several other sites that were listed and none of them had much to say. The above reference website has illustrations of both the mop and the metal bending machine and a short history of how and why then were invented. The article doesn't say much about Mr. Stewart, though, except that he was African American and was born in the 1800's.