functions.php

<?php
/**
 * Exclude 'Behind the Scenes' category from blog
 * @author Bill Erickson
 * @link http://www.billerickson.net/customize-the-wordpress-query/
 *
 * @param object $query
 */
function be_modify_blog_query( $query ) {
	if( $query->is_main_query() && $query->is_home() ) {
		$tax_query = array(
			array(
				'taxonomy' => 'category',
				'field' => 'slug',
				'terms' => 'behindthescenes',
				'operator' => 'NOT IN',
			)
		);
		$query->set( 'tax_query', $tax_query );
	}
}
add_action( 'pre_get_posts', 'be_modify_blog_query' );