is_blog_page.php

<?php
/**
 * There is not a hard and fast rule to check if the 
 * current page is the blog page (the page you set in 
 * the settings to display blog posts). This is what the 
 * documentation says: "There is no conditional tag for the blog page."
 * 
 * Read the full section here: https://codex.wordpress.org/Conditional_Tags
 *
 * Not sure if there is another way to achieve this!
 * 
 */

/**
 * This function has been tested when a 
 * specific page has been set to display 
 * posts and a static page is set for the 
 * Homepage in "Settings > Reading: Your homepage displays"
 * @return boolean
 */
function is_blog_page() {

  if ( is_front_page() && is_home() ) {

    // Default homepage
    $is_blog_page = false;

  } elseif ( is_front_page() ) {

    // static homepage
    $is_blog_page = false;

  } elseif ( is_home() ) {

    // blog page
    $is_blog_page = true;

  } else {

    // everything else
    $is_blog_page = false;

  }

  return $is_blog_page;

}

There is not a hard and fast rule to check if the current page is the blog page (the page you set in the settings to display blog posts). This is what the documentation says: "There is no conditional tag for the blog page."

Read the full section here: https://codex.wordpress.org/Conditional_Tags