I've had a Blue card for many years. Since there's no annual fee, it serves as a convenient place holder for MR points when I don't want to keep a Gold or Plat card.
If you keep your PRG card then all points earned by using either card will be considered to be regular Memebership Rewards points. If you cancel your PRG card and keep just the Blue card, then all your points will convert to Membership Rewards Express points, which are not as useful (they cannot, for example, be transferred to airline FFPs or hotel guest programs). If you subsequently get a new Gold/Plat card, then all you points will magically convert back to MR/MR First points again.
Makes for a great card to have if you plan to churn.
Also, I've noticed that on the AMEX website there are more "Special Offers" that show up for my Blue card than either my Gold or Plat. Small ones to be sure, but a painless way to save a couple bucks here and there.