The Quick Version

Copy this code:

<script>
(function() {
    Array.from(document.querySelectorAll("a")).filter(a => {
        return a.hostname && a.hostname !== location.hostname
    }).forEach(a => a.setAttribute('target', '_blank'))
})();
</script>

Head to your Ghost settings, Code injection, paste it into site footer and hit save:

Code injection settings for a Ghost blog

Done! (Might need to do a hard refresh or wait until the cache is cleared to see the change).

This opens links to your own blog in the same window/tab, but links to other websites in a new tab/window.

The Longer Version

So this is a vanilla version of this popular post:

I want my external links to open in a new tab
Hello all I’d like all external links on my blog to open in a new tab. I’ve been reading the blog posts on this forum from people that had a similar question. I found some possible solutions that were posted here, but none of them seem to work on my Ghost website. The current version of Markdown s…

I changed two things about it:

  1. Changed it to an IFFE
  2. Made new tabs open in the (normal/default) _self context
  3. Made it vanilla

But, for the really old browser Internet Explorer it will quietly not work. Because old IE doesn't work with Array.from(), see more at MDN Web Docs. Just for the very small amount of users (<2% as of 2020) who might visit your site, they won't get the luxury of new links in another tab.