How to add custom post meta to default blog post?

You can use the action hook save_post_{post_type}, so for the post type ‘post’ (blog posts) it is: save_post_post. For more info see the codex.

add_action( 'save_post_post', 'sw150216_save_reading_time', 10, 3);

function sw150216_save_reading_time( $post_id, $post, $update ) {
    $reading_time = estimated_reading_time( $post );
    if ( $update ) {
        update_post_meta ( $post_id, 'reading_time', $reading_time );
    } else {
        add_post_meta( $post_id, 'reading_time', $reading_time, true );
    }
}