WordPress' missing is_blog_page() 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.

is_blog_page.php

<?php

/**
 * WordPress' missing is_blog_page() 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.
 *
 * @see http://core.trac.wordpress.org/browser/tags/3.4.1/wp-includes/query.php#L1572
 *
 * @return bool
 */
function is_blog_page() {
    
    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 (
        ( is_home() || is_archive() || is_single() )
        && ($post_type == 'post')
    ) ? true : false ;
    
}