I thought that when the Delta blues was developing mainly among African Americans in the Mississippi Delta cotton area the fiddle was not a particularly common instrument in those settings. Fiddle was more of a Euro-based eastern and southern mountain instrument. Instruments for early blues would have been mainly aimed at just accompanying the singer, so having an expressive solo instrument wasnt initially a major consideration. Banjo and other more traditional African American instruments of the mid-late 1800s would presumably have predominated in the Delta agricultural workers blues until the end of the century when the steel string guitar (sold in catalogs) came in, and then piano and band instruments once the City based composers got into the act .   Piedmont blues, which developed a little later and much closer to the Appalachians is raggier and may have had more fiddle involvement in its development( like the Pig Ankle Rag perhaps?).   There are certainly some blues which are frequently played on fiddle - and, just to prove generalizations, are dangerous one of the most popular - Carroll County Blues - was "written" by a fiddler from central Mississippi - albeit well after 1900.   It is an instrumental though.