My opinion is that as a general rule Amex does blacklist until you pay them off. But like other posters have mentioned there are some people who fall through the cracks. I think those are more of an exception not the rule.
You could go ahead and apply. If you are blacklisted it would only be a soft pull.
sjt, AFAIK it is not even a SP. AMEX's database will recall the previous file/record and not even SP. Ususally results in a declined app with no pull.