div elements don't receive focus by default, so you can't register an
onblur event unless you first add
contentEditable (see StackOverflow thread). Without this, I'm not sure your function will execute in a browser, let alone in Jest.
Side note: the convention here is to use lowercase
onblur vs. camel-case
onBlur, although both technically will work.
Now more to the point: I have had success calling
blur() in Jest on an element that has focus. If your element is able to receive and lose focus, you should be able to do this:
let container = element.shadowRoot.querySelector('div.container');