I think of a blender fabric as one that has a subtle print design and the colors that link together the main fabrics that you have chosen for a quilt. They are not solids, but have at least two or more different hues or shades of colors that help 'blend' with and make the transition from one bolder fabric to another. They usually don't 'stand out', but function as a background and lend to the overall look, making a cohesive design. Batiks are often used as blenders because they have several colors which 'blend' smoothly together, and you look for ones that complement the colors in your main focus fabrics. Other blenders are 'tone on tone' or small print fabrics like ManiacQuilter shows that serve the same purpose of transitioning from one focus/bold fabric to another.