I’m running into this same issue. I set the CPT to support comments. I then look at the TwentyNineteen theme and inspected the single.php file where I found this bit of code:
// If comments are open or we have at least one comment, load up the comment template.
<?php
if ( comments_open() || get_comments_number() ) {
comments_template();
}
?>
I copy/pasted that into my single-blog.php file. But my next problem is that the “Discussion” section is not visible in the WP editor. I went to Screen Options and it doesn’t show up.
This seems to be a frequent problem, would really appreciate clear directions that could be added to your documentation.