Using custom mod_rewrite without breaking wordpress permalinks

You don’t have to touch the Apache configuration for this, you can do this all from the WordPress rewrite system. But indeed, your general idea is correct: you should append ?cat=3 (or category_name=blog if you want to be even clearer). Remember that your blog archive can have multiple pages, maybe you want to handle this too.

This is how I would do this from inside WordPress:

add_action( 'init', 'wpse7379_init' );
function wpse7379_init()
{
    add_rewrite_rule(
        'blog(/page/([0-9]+))?/?$',
        'index.php?category_name=blog&paged=$matches[2]',
        'top'
    );
}