This function will re-introduce the default content WYSIWYG textarea to the designated static blog page in WordPress that was removed in 4.2.2. Add this code to your functions.php file in your theme.
if( isset(…page_for_posts’) )
This first check is to see if the post object is set and that the ID matches the ID of the set static blog page. If either of these conditions are not met, the function returns nothing.
This function removes the messages about this being the posts page that appear after the title. The ‘edit_form_after_title’ hook literally refers to editing the form as it appears after the title (source), and the ‘_wp_posts_page_notice’ parameter refers to the notice displayed when editing the page for posts (blog index).
This adds the editor back to the page.