I had a similar problem with wide backing fabric I bought at Joanns - I think it was Quilter's Legacy. But it had a black line the entire length of the fold. I didn't catch it until I was serging the ends so that I could wash it. I was sure I was stuck with it, since I had started the serging, but I took it back anyway. They checked the bolt, and the rest of the bolt was also flawed. They ordered a new bolt, and gave me new fabric.
Given that you have the black line flaw, and the odd threads in the weave, I would take it back to the shop.