Blucher vs Derby

[Churchill W]

OK, so I am so far convinced that balmorals and oxfords are not interchangeable contrary to what some people argue is just a differentiation between what Americans and the British call it. In the Carmina thread , a member argues that Derbys and Bluchers are also not interchangeable:

Actually the Derby and the Blucher are two different shoes. there is a subtle, yet important detail in the uppers (info here http://en.wikipedia.org/wiki/Blucher_shoe) But look at the difference in the cuts of leather between the following two shoes The top is a blucher and is made from a single piece of leather, where as the bottom is a derby and typically made from 3 pieces (excluding the tongue). Derby is originally a British shoe style, and blucher is Prussian. Why exactly America has adopted blucher to refer to all open throat shoes, not quite sure.

Can the experts weigh in on this?