To me blenders are fabrics with prints that have low contrast - almost tone on tone - but with "movement" in the print. "Movement" is a print that has shapes which are usually random, with curving lines. As much as I love "Civil War" fabrics, they are not usually what I consider as "fabrics with movement." Marbles are also great, but are not necessarily "moving". I do use them as blenders, though.
My theory, with simple designs, "if you're not doing the work the fabric has to." (nod to Mary Ellen Hopkins)
Jan in VA