Well… You’ve already used wp_enqueue_style. Use it again. That is the canonical mechanism for loading stylesheets either by itself or in combination with wp_register_style
wp_enqueue_style('blackberry',get_stylesheet_directory_uri().'/path/to/blackberry.css');
I feel compelled to note that user agent sniffing is not especially reliable. Is it not possible to do this with media queries?
Reference
http://codex.wordpress.org/Function_Reference/get_stylesheet_directory_uri