If you are comfortable working with the WP_query on your blog template it is an easy parameter to add:
http://codex.wordpress.org/Class_Reference/WP_Query#Order_.26_Orderby_Parameters
If you have any trouble with this post back the name of the theme you are using and I will try to help you find where to put the code.
you could try to work with pre_get_posts – http://codex.wordpress.org/Plugin_API/Action_Reference/pre_get_posts
this example code, to be added into functions.php of your theme, should reverse the order of the posts in the index and archive pages etc:
function inverse_post_order( $query ) {
if ( !is_admin() && $query->is_main_query() ) {
$query->set( 'order', 'ASC' );
}
}
add_action( 'pre_get_posts', 'inverse_post_order' );
