IF you cannot return it and as a last resort, take the fabric to a good light and find the very end/beginning of the dark line. See if you can carefully lift it out of the thread, with the point of a fine needle. I once found a dark line, in a piece of older flannel fabric. Figured I had nothing to lose, so started trying to pull it out . . . and did! It ended up being an extra fiber, that had somehow been woven into the fabric. I was able to pull out a small section, cut, then pull out another small section, etc. I was careful not to pick up the actual flannel thread and in the end, that thread had just a slightly looser weave. After a few more washes, it was no longer noticeable. It was a PITB and seemed to take forever . . . but it did save the piece of fabric.