Hi Mak,
Someone moved your question to Windows Store app dev forum, but personally I don't think it is a Windows Store app question, and I don't know if it is too late to provide any suggestion.
The possible solution is to install the
latest update for VS2012, the idea comes from here:
http://stackoverflow.com/questions/14175497/wpf-blend-of-visual-studio. Everything works fine in my environment.

Best Regards,
--James
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Thanks
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