Since I assume you’re using wp_nav_menu(), there should be a class current_page_parent which is applied to your posts’ page menu item when viewing a single post.
You can use this class to style the ‘active’ state, much like you’re probably doing with current_page_item at the moment.
If (for whatever reason) you must have current_page_item added too, you can filter the nav_menu_css_class:
/**
* Add the class 'current_page_item' to 'page for posts' menu item if we're
* viewing a single post.
*
* @param array $class
* @param object $item The current menu item.
* @return array
*/
function add_current_class_to_posts_page( $classes, $item )
{
static $posts_page;
if ( ! is_single() )
return $classes;
if ( ! isset( $posts_page ) )
$posts_page = get_option( 'page_for_posts' ); // cache as we may be calling this a lot!
if ( $item->object == 'page' && $item->object_id == $posts_page )
$classes[] = 'current_page_item'; // this is the posts page!
return $classes;
}
add_filter( 'nav_menu_css_class', 'add_current_class_to_posts_page', 10, 2 );