Views block link defaults to target="_blank" using rewrite output
Print the results from the views template.you can rewrite the views template for that specific field and check the link that whether its external or internal using the drupal function url_is_external and you can add the attribute target="_blank" to those views fields that are external urls.

Off topic suggestion : if you want all external links on your website should open in new tab then you can use the extlink module to open all external links in new tabs.


  External Links is a small module used to differentiate between
  internal and external links. Using jQuery, it will find all external
  links on a page and add an external icon  indicating it will take you
  offsite or a mail icon  for mailto: links.