Gaussian blur effect for video clips and images

For the purposes of focusing the viewer's attention on overlaid titling or overlaid transparent images, it would be cool to be able to darken and gaussian blur any part of a video clip or image on the timeline. (the idea is to have the darkening/blurring options available from the Callout menu ... but without having to create a callout.) What would be ideal is if the blur effect was a separate track on the timeline that would blur all layers beneath it. But that seems like it might be challenging.