this is taken from the AMEX BLUE t&c page it sounds to me like all purchases are eligible for %2 extra if you carry a balance:
"Your annual cash reward for everyday purchases at stand-alone U.S. supermarkets, gas stations, drugstores, and home improvement stores is up to 3%. Stand-alone locations do not include the departments of superstores or warehouse clubs where the standard rebate of up to 1.5% applies. Your annual cash reward for all other purchases is up to 1.5%. You will receive an additional rebate of up to 2% in months in which your account carries a balance. Your cash rewards are limited to $50,000 of annual eligible spending. Balance transfers, purchases of American Express® Travelers Cheques or American Express® Gift Cheques and Cash Advances do not earn a cash back reward."