Richard ~ I'm sure that "Spanish tuning" has different meanings in different cultures. The guitar that we know today, after all, has been called the "Spanish guitar," so I'm not surprised that some folks might describe the standard tuning of the instrument as "Spanish."

The usage I was decribing is something I not only have read in books about the blues; I also heard it straight from the mouth of Mississippi Fred McDowell one memorable evening in 1969.