How can you render fields from an entity reference in node templates? [Drupal 8]
#node there is actually the current node.

If you want to access fields and their values, start off with node, not content, which is a render array with the configured fields.

You can access the reference entity like this {{ node.field_blog_author.entity }}.

From there, you can continue again to the field of the referenced entity that you want to display. You could try calling the view method, with will view it with the default settings, you can also pass a view mode as a string to it or an array display settings (not sure about the exact syntax for that in twig).

Also, my standard reference, see http://wizzlern.nl/drupal/drupal-8-entity-cheat-sheet for a great overview of content entities, how they're structured and how to access values. Most of that also applies to twig.

And last, when trying to figure out what's in a content entity, use node.toArray(), that gives you an array representation that's very close to how you can access it as an object. While the internal structure is quite different.