An is_blog() conditional function for Wordpress.

is_blog.php

<?php

/**
 * An is_blog() conditional function for Wordpress.  
 * 
 * Determines if the current page is blog(y), including:
 * 
 * - Posts page
 * - Archive (category, tag, author)
 * - Single post
 * - Custom page template that may be displaying blog like content (like a custom archive page)
 *
 * @return bool
 */
function is_blog( $templates = array() ) {
  global $post;

  $post_type = get_post_type($post);

  return ( 
  	in_array( true, array_map( 'is_page_template', $templates ) ) || 
  	( is_home() || is_archive() || is_single() ) && ( $post_type == 'post' || ! $post_type ) 
  );
}