Open “Visit site” in a new window?

by

This should do it (put it in a network activated plugin):

add_filter( 'myblogs_blog_actions', 'my_sites_visit_site_target_blank', 10, 2 );

function my_sites_visit_site_target_blank( $actions, $user_blog ) {
    $to_replace = ">" . __( 'Visit' ) . '</a>';
    $to_replace_with = " target="_blank" >" . __( 'Visit' ) . '</a>';
    return str_replace( $to_replace, $to_replace_with, $actions );
}