Thirty years ago we used 50/50s if they felt right. That being said, I would wash everything going into to quilt top or the back in exactly the manner the completed quilt will be washed and dried. Even blends get stretched as they are put onto the bolt and will sometimes change slightly.
Personally, I would also avoid triangles as blends don't conform to a new size as easily if you have to wiggle and jiggle one of the fabrics to make points and corners perfect. They just don't have as much ease. Other than that, have at it.
I have a 50/50 from the 90s that I want to use somewhere. It won't be for a back because it isn't particularly soft, but it is a good weight and very pretty. One plus is that a blend makes for a durable fabric.
Next year when cotton goes up in price, blends may look very apppealing.