Download data from URL with GWT

This is a typical img with src having a blob:

    <img class="gwt-Image" src="blob:a7cb8111-cf35-4c3a-8295-bdda0ff66caf">

Is there a way for my GWT app to download get his blob data for client side manipulation?

I've tried this: 

    private native String blobToBase64(String source)/*-{

var xhr = new XMLHttpRequest();

xhr.open('GET', source, true);

xhr.responseType = 'blob';

xhr.onload = function(e) {

 if (this.status == 200) {

   var myBlob = this.response;

   alert("Converted to Blob");

 }

};

xhr.send();

}-*/;

However, alert box does not show up when passed the blob source, so it does not work. And the `this.status` response is `0`

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