According to what you mentioned your QA site is on www.mydomain.com
and wordpress site is on www.mydomain.com/blog
So you have to follow the same stpes what mentioned in docs.. However make sure WordPress should installed first than Q2A
So install Wordpress and than setup wordpress path in qa-config.php as below
define('QA_WORDPRESS_INTEGRATE_PATH', '/blog'); //this path is important /blog you may need to check your path if it doesn't work
And set database name, userid and password (it should be WordPress db)