Specify custom behavior on blur event.
A [blur event](https://developer.mozilla.org/en-US/docs/Web/Events/blur) fires when
an element has lost focus.
Note: As the `blur` event is executed synchronously also during DOM manipulations
(e.g. removing a focussed input),
AngularJS executes the expression using `scope.$evalAsync` if the event is fired
during an `$apply` to ensure a consistent state.