I downloaded the source and took a look, I see that Disqus hooks onto the comments_template
filter so you can try this;
//hook onto comments_template filter with priority 1
add_filter( 'comments_template' , 'disable_disqus', 1 );
function disable_disqus($file) {
//if not blog page, remove disqus filter
if( !is_page('blog') ) {
remove_filter('comments_template', 'dsq_comments_template');
}
//returns regular comments template
return $file;
}