TVML background not updating when image updates

Question

When a "product image" (a.k.a. the heroImg) is first displayed in my ProductTemplate TVML layout, the background is automatically set to a blurred copy of the image.

The image itself is updating when I ask it to:

function changeHeroImage(incomingString) {
    if (incomingString) {
        var theHero = myDoc.getElementsByTagName("heroImg").item(0);
        theHero.setAttribute('src', incomingString);
    }
}

.. but not the background; the layout retains the background from the original image. Any idea on how to force the background to update, to reflect the current image?