Not exactly a myth, but:
As I understand it, a kasumi finish is the result of grinding through a rough forged finish, so a kurouchi blade will be black at the top, gray kasumi towards the middle, and naked steel towards the edge--as in this thread that Jon contributed to:
kurouchi
What I don't understand is faux kasumi finishes. At 2:25 in this video a guy is using a jig to apply a faux kasumi finish. Is this the way it's usually done? Is he using a stone or something else?