I came across this similar issue when displaying blog posts in a summary block. If the "Link Post Title to Source URL" option is selected, there is no way to make the link open in a new tab.
I found a solution in another post on this form that is counter intuitive but seems to work. Adding "target="_blank to the end of the Source URL will add the target="_blank" to the html. For example: https://link-to-outside-source.com"target="_blank Yes, I know, the code looks wrong if you are use to writing html, but try it. Plus, no JS is required for those on the lower level subscriptions.