rgba background with IE filter alternative: IE9 renders both!

I'm trying use make a div's background transparent using a mixture of CSS3 rgba() and microsoft's filter property like this:

div {
    width: 200px;
    height: 200px;
    /* blue, 50% alpha */
    background: rgba(0,0,255,0.5);
    /* red, 50% alpha */
    filter: progid:DXImageTransform.Microsoft.gradient(startColorstr=#7FFF0000,endColorstr=#7FFF0000);
}

As expected, browsers that support rgba() will render the div as blue, whereas IE 6-8 will render it as red.

IE9 can apparently handle both (previously I thought filter support had been removed) and the result is a purple div. Is there any way of making IE9 support either of these properties but not the other? rgba() would obviously be preferred.

N.B. I am using IETester to run these tests. If the proper build of IE9 does not do this please let me know.