Edit in JSFiddle

angular.module('app', [])
.filter("hashtag", function($sce) {
    return function(text) {
        return $sce.trustAsHtml(text.replace(/#([^\s]+)/g, "<b style=\"color:blue\">#$1</b>"));
    }
})
.controller("MyCtrl", function($scope) {
    $scope.msg = "hashtag #hello #world bye";
});
<html ng-app="app" ng-controller="MyCtrl">
    <label ng-bind-html="msg | hashtag"></label>
    <br>
    <input type="text" ng-model="msg" />
</html>