The 'quilt blocks as signals or signs on the Underground Railway' story is a nice one, but it is fiction and needs to be recognized as such. I have nothing against a good story, but do become concerned when the story purports to be historical fact but isn't.
Here are just a couple of interesting references:
http://historiccamdencounty.com/ccnews11_doc_01a.shtml
and
http://www.lacwrt.org/quiltcode.html and, of course, Barbara Brackman's book, "Facts and Fabrications; Unraveling the History of Quilts and Slavery" (C&T Publishing)