The Back Button was designed for navigation within the same browser tab (like the browser's own back button).
However, there are a couple of things that you could do.

(1) Use the Gallery Title (instead of the Back Button) as a link using HTML formatting as noted in this FAQ.
How do I add HTML formatting to the Gallery Title or Back Button?
You could perhaps use a Gallery Title such as:

<a href="http://www.example.com" target="_blank">Click here</a>

(2) You could dynamically add the target="_blank" attribute to the Back Button's <a> tag using JavaScript and the Juicebox-Pro API as follows. (The onInitComplete() method is used to ensure the .jb-go-back class is present in the DOM before the target="_blank" attribute is added.)

<!--START JUICEBOX EMBED-->
<script src="jbcore/juicebox.js"></script>
<script>
    var jb = new juicebox({
        containerId: 'juicebox-container',
        backButtonPosition: 'TOP',
        backButtonUrl: 'http://www.example.com'
        
    });
    jb.onInitComplete = function() {
        $('.jb-go-back').children('a').first().attr('target', '_blank');
    };
</script>
<div id="juicebox-container"></div>
<!--END JUICEBOX EMBED-->