Don't order WindowManager to do something here. I think you have |attentionscreenwillshow| event ? If not, create one.
Then do the blur in WindowManager.
```js
  window.addEventListener('attentionscreenwillshow', function() {
    var app = runningApps[displayedApp];
    app.blur();  // We don't have this yet, implement it in window.js
  });
```