The searing and beautiful portraits in the collection “ Blue Muse: Timothy Duffy’s Southern Photographs ” seem born of a bygone era. Indeed Duffy made his pictures using a Civil War-era technique called the wet plate tintype that limited his shoots to just one or two frames per day. But his subjects, or collaborators as he calls them, are accustomed to sweating under bright lights and performing for an audience. They are the blues, gospel and roots musicians Duffy has been recording his entire