I've been having similar issues. In Expressions Blend 4 RC, I can no longer build the project without errors. The <UserControl> tag ...the very first line in the XAML...has an "Error HRESULT E_FAIL has been returned from a call to a COM component" error. If I mouse over the curly line, I get some more info like
at MS.Internal.XcpImports.CheckHResult(UInt32 hr)
which, sadly, doesn't mean that much to me.