Just as FYI. There were two houses in my area used for the undergraound railroad. While it is not known if a quilt was used as a signal, one had a quilt survive from that era. But the story told most was that they used words set to familiar songs to convey the trail route. That being true, why would a quilt not be believable? Hobo's used symbals on fences in later decades to show what was lurking at different houses. A quilt could've been used. Maybe it wasn't the block in the quilt, but that there was a quilt hanging was the signal.
Worse myths have survived.