Did you set these constants in wp-config.php? Btw. the plugin constants are optional.
On my development instance, I’ve set the following before ABSPATH and it works without any problems:
define('WP_CONTENT_DIR', $_SERVER['DOCUMENT_ROOT'] . '/blog/wp-content');
define('WP_CONTENT_URL', 'http://' . $_SERVER['HTTP_HOST'] . '/blog/wp-content');
Limiations:
a) If you use SSL, you should also check for the rigth protocol with is_ssl().
b) Not every plugin in the repository is using these constants. Many plugins out there has hardcoded paths relative to the root directory. So you could run in some problems if you really need a thirtparty plugin.