To add the Blog to the start of all blog posts, you need to add it in permalink page:
In the Permalinks settings (WP_Admin/ Settings /Permalinks), select “Custom Structure” and add the /blog at the start of the input field:
If you have custom post type, you need to make sure that “‘rewrite with front’ is disabled. to do that go to the CPT declaration code and add/update the rewrite:
“rewrite” => array('with_front' => false)
If you using 3rd party plugin, you can find the code on plugin page and add it into your functions.php page to override it.