Thanks for the suggestions, everyone. I had no luck at all with vinegar, nor with cream of tartar. Being in Canada, it will cost me more to ship the Blue Line eraser than the product costs
Plus, I want to get this fixed now, not in 4 week when the mail arrives. But I may still invest in some of that for future use.
But in my googling I found a mention of using hairspray. Seems odd, but what do I have to lose. So I tried it on a small section. It didn't look like it was working at first, but when I rinsed the quilt the marks appear to be gone! Waiting now for that section to dry so I can double check, but I might have a solution in hand. Will update when I know for sure.