You have to add it to content.php. The reason you add it here to add it to all your blog posts, is that it follows a hierarchy of templates to call the content.php into every blog post. It goes Main Index Template(index.php) > content.php. Meaning content.php is part of the Main Index Template.
So if you want to add your field right under the title, go to content.php, and add your php snippet under a div tag classed “entry-content”. Just add it under that opening tag. 🙂
