is_blog.php

/**
 * WordPress' missing is_blog() function.  Determines if the currently viewed page is
 * one of the blog pages, including the blog home page, archive, category/tag, author, or single
 * post pages.
 *
 * Doesn't include is_search(), if you only use search for the blog, then add that in or test separately
 *
 * @props grantnorwood @wesbos @toscho @jjeaton
 * @return bool
 */
function is_blog() {
    global $post;

    // Post type must be 'post'.
    $post_type = get_post_type( $post );

    // Check all blog-related conditional tags, as well as the current post type,
    // to determine if we're viewing a blog page.
    return (
        ( $post_type == 'post' ) &&
        ( is_home() || is_archive() || is_single() )
    );
}