I use to produce them in the late 80's, along with MBK's and I think it should be easy to spot a cross with a MBK. I never produced a Mel. Thayeri with ANY white at all. There was always a detectable pattern with in the black. They babies I produced always showed a milksnake phase pattern, even though it was black on black. I've spoken to other guys who have said theirs showed the leonis pattern, but I've never seen it.
Also the smallest MBK hatchling should be close to twice the size of a Thayeri.
The reason I dumped them is that it was difficult to move a fickle eating tiny black snake for $150, when people could buy a voracious eating MBK for one third of that.
They may be better now, with forums like this where you can reach people who can appreciate the difference between the two. But the casual show going buyer looking for a bargain wouldn't.