Build fails when targeting .NET 3.5 with BlurBitmapEffect class

I am building a project that targets .NET 3.5 and uses the BlurBitmapEffect class.  I am using Visual Studio 2013.  This project is used with 2 solutions, one that includes only projects that target .NET 3.5 and another that also includes projects that target .NET 4.0.  When I build either solution (or if I build the project alone), I get the following error: The tag 'BlurBitMapEffect' does not exist in XML namespace 'http://schemas.microsoft.com/winfx/2006/xaml/presentation'.

I understand that the BlurBitmapEffect class is obsolete in .NET 4.0 and higher, but why would that keep a project targeting .NET 3.5 from building?  Is there another setting I'm missing?