// Category should be 'cat'
<?php
$args = array( 'cat' => $cat, 'orderby' => 'post_date', 'order' => 'DESC', 'post_type' => 'blog', 'post_status' => 'publish' );
?>
// Category should be 'cat'
<?php
$args = array( 'cat' => $cat, 'orderby' => 'post_date', 'order' => 'DESC', 'post_type' => 'blog', 'post_status' => 'publish' );
?>