Wordpress function to get the URL of the posts index page if this is not the home URL.

page-for-posts-url.php

/**
 * gets the link to the article index
 *
 * @global $blog_id
 * @return string
 */
function get_page_for_posts_url() {
	global $blog_id;

	$index_id = 0;
	if ( is_multisite() )
		$index_id = ( int ) get_blog_option( $blog_id, 'page_for_posts', 0 );
	else
		$index_id = ( int ) get_option( 'page_for_posts', 0 );

	if ( 0 === $index_id )
		return get_home_url( $blog_id );

	return get_permalink( $index_id );
}