After more digging I discovered that the uninstall I mentioned for VS 2013 RC must have also uninstalled the Expression Blend SDK for .Net 4.
Oddly enough the Dll's that I had referenced were leftover from the 3.5 SDK but the namespace was for the .Net 4 SDK and had always been working. (Until that SDK was uninstalled). A clue for me was when I went to enter another namespace and the "//schemas.microsoft.com/expression/2010/interactivity" selection was no longer in the intellisense list.
I reinstalled the Blend SDK for .Net 4 and all is well again. Thanks for the attempts to help. A reminder to me to always focus on the question "what has changed" even though it may not be where you suspect...