try this instead:
<button id="addLine" type="image" class="AddButtonClass" title="Add New Line" data-id="{{blockId}}"></button>
and in your backbone for example in your initialize function:
$('#addLine').click(function(){
alert($(this).data('id'));
});
or you can create an event in backbone
events: {
'click #addLine' : 'addNewBlock'
},
addNewBlock: function(e){
console.log( $(e.currentTarget).data("id"));
}