// There isn't a function built-in for this for many reasons, but it can be done with javascript like the example below:
function wpex_add_target_blank_to_portfolio_entries() { ?>
<script>
( function( $ ) {
'use strict';
$(document).ready(function() {
$('.portfolio-entry a').attr( 'target', '_blank' );
});
} ) ( jQuery );
</script>
<?php }
add_action( 'wp_head', 'wpex_add_target_blank_to_portfolio_entries' );