ng-blur is not called from a function triggered by ng-keyup

so I have a ng-blur function that correctly fires if I do this

  $scope.disableReturnButton = function($event){
    document.onkeyup=function(e) {

        if(e.which == 13){

          $event.target.blur();
          return false;
        }
    }
  }

however if I try to set the keyup event on the element only, it doesn't seem to trigger ng-blur when the blur event happens

  $scope.disableReturnButton = function($event){

    if($event.which == 13){
      $event.target.blur();// this blurs the element but doesn't trigger the ng-blur
      return false;
    }

  }

here's the markup

<span contenteditable="true" class="blanks" 
  ng-blur="blurredWhy($event)"
  ng-focus="focusedWhy($event)" 
  ng-bind="data.why" 
  ng-keyup="disableReturnButton($event)"></span>