opening external web page from web app causing web app to go blank

I've just wrapped my web site into a web app so it can be offered in the App Store for iOS. On my site, I have a number of links to external web pages, all with target="_blank" in the a link. I have the following code on the page that contains the links and all links are properly opening in a separate Safari browser window, but my app shows a completely blank page after the link is tapped. How do I fix this?

$(document).on('click', 'a[target="_blank"]', function(ev) {
    var url;
    ev.preventDefault();
    url = $(this).attr('href');
    window.open(url, '_system');
});