Apply multiple custom classes to the body tag with the body_class filter.

functions.php

<?php
//* Do NOT include the opening php tag

add_filter( 'body_class', 'ck_custom_body_classes' );
function ck_custom_body_classes( $classes ) {
	
	//* Create an empty value (or add a default to be applied everywhere)
	$classes[] = "";
	
	if ( is_front_page() ) {
		$classes[] = "front-page";
	}
	
	if ( is_page_template( 'page_blog.php' ) ) {
		$classes[] = "blog-page";
	}
	
	return $classes;
}