I am afraid that I can not reproduce this exception. Also, given the fact that it only occurs in blend is indicative that it might be its fault. Also, the exception states that it cannot convert a type to a type with the same name which may be due to an assembly mismatch. Why Blend loads two different versions of the same assembly is anyone's guess.
You can try uninstalling any previous versions of the controls and install just the latest release in a clean environment.
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