Say what?

I had no idea where the expression "It's a bit black over Bill's mother's" (or, in my experience, "It's a bit black over Will's mother's") came from, but this BBC article provides a helpful explanation - as it does for several other curious and baffling regionally specific phrases. However, it doesn't explain how an expression whose usage was supposedly restricted to the Midlands was frequently employed by family members in the north-east, and whose roots are in Bedfordshire.