Now that's exclusivity

Can you name the iconic luxury brand that began life as an urban legend and only later became a reality?

The answer is the American Express Centurion card or, to use its more colloquial title, the Black Amex. During the 90s, a rumour spread about a fabled black Amex card that had an unlimited credit limit and was available only to an ultra-rich, uber-exclusive membership. The rumour became so prevalent and powerful that American Express eventually decided to capitalise on it. In 1999, it launched the Centurion card, by invitation only, to its highest-spending and biggest-earning customers.

To qualify for the Black Amex, customers must spend about £100,000 a year on their existing Amex, have a perfect credit record and travel extensively. If the member is offered the card and agrees to pay the annual £650 fee, they receive the titanium card in a mahogany box. The additional benefits of the Black Amex are almost as legendary as the card itself. Aside from an unlimited credit limit, cardholders can enjoy a complimentary business-class ticket for their partner whenever they fly, a personal 24-hour concierge service, free personal shoppers in many of the world's major department stores and additional free nights of accommodation at top hotels - simply by paying with their cool black card.

The card has, not surprisingly, attracted a stellar membership. Celebrities such as Nelly, Jay-Z and Paris Hilton have all recently name-checked the card in music and film. Kanye West even included his experiences with the card in a recent song: 'She was like, "Oh my God, is that a black card?" I turned around and replied "Why yes, but I prefer the term African American Express"'.

Most marketers can only dream of this kind of celebrity endorsement and buzz. But the Black Amex's ongoing success depends on it maintaining its ultra-exclusive membership and secretive profile. If the card becomes ubiquitous, it risks losing its target market and the partner companies that offer freebies in return for access to the world's elite customers.

Amex has not helped the situation. A direct marketing blunder last year saw a top-secret mailing sent to 250,000 regular Amex customers, rather than the 10,000 Black Card holders it was intended for. Amex has also got sloppy with its websites, with many openly revealing the existence of the card. Worse still, it recently featured in the most mass-marketing vehicle of all - a Bond product placement. When Daniel Craig's Bond checks into The Ocean Club in Casino Royale he pays for his room with his Black card.

The greatest strength of the card, initially at least, was that most people did not know it existed. It does not get any more exclusive than that.

Take the British Airways Black Card, for example. BA has three tiers in its frequent-flyer programme: blue, silver and gold. But above gold, and in almost complete secrecy, there is also a black card, reserved for the 200 customers BA deems to have 'status'. If you are the chief executive of a firm whose employees fly a lot with BA, a key member of parliament or a super-celebrity, the card is yours. Owning the BA black card means you no longer need to worry about customs or seat reservations; you will be whisked straight to First Class. If you are running a little late, BA can hold the plane back for you.

Unlike Amex's efforts, BA has got it right. A card with exclusive benefits, available to only the select few, and an amazingly secretive service that no one in the general market even knows about. Oops.