According to this blog (http://demosthenes.info/blog/534/Cross-browser-Image-Blur-with-CSS) the blur filter was dropped after IE9:
filter:progid:DXImageTransform.Microsoft.Blur(PixelRadius="3");
They did give a solution called StackBlur (using JavaScript and Canvases):
http://quasimondo.com/StackBlurForCanvas/StackBlurDemo.html
It is in the form of a javascript add-on downloadable from that site.