I did some digging, and found out much more about this literally mythical group of horsemen.
Well, maybe not entirely mythical, but certainly different than portrayed in several sources, including letters and diaries of the Fire Zouaves themselves.
The Black Horse was initially formed in Faucquier County, VA, in 1859. It was an idependent volunteer cavalry company--a mounted militia.
It was made up of young (and probably some not-so-young) men from the oldest, richest, most established families in the county, which is in the heart of Virginia's horse country. These knights gained their equestrian skills while fox hunting, and participating in actual "jousting tournaments" held in Faucquier Springs.
The Black Horse was mustered into the service of the Commonwealth of Virginia in May, of 1861. Therein, it became Company H of the 4th Virginia Cavalry. The 4th Virginia was an elite group, made up of companies from Prince William, Chesterfield, Madison, Culpepper, Powhatan, Goochland, Hanover, Warren, and Buckingham counties--tidewater counties all, and wealthy enough to support these fine sons of Virginia in style.