Jim, I took a look through my copy of H. Grady Howell, Jr.'s For Dixie Land I'll Take My Stand! Volume IV - Images of Mississippi Confederates. Looking only at the military uniforms (not chivilian), I see many coats that are quite dark, possibly black, but I found myself in need of a definition of a "frock coat." For example, does a frock coat have lapels or no lapels, if lapels, wide lapels or narrow lapels, a collar or no colar, one vertical column of buttons, two vertical columns of buttons or no buttons, etc. ... ? What you are looking for might be there, but I found I did not really know from your description what I was looking for.