Thanks to this thread, I applied for the (old) Blue Cash over the phone and was approved. I'm a current Amex cardholder and when I was transferred to the Blue Cash team I explained I was calling specifically for the Blue Cash from American Express with the 5% cash back after $6,500, not the Everyday or Preferred cards.

The representative also confirmed that the $6,500 threshold can be met by purchasing any goods/services totaling $6,500. So, if one were so inclined, you could kick in the 5% by heading down to the Apple Store and swiping for a new Mac Pro, then stopping by Whole Foods on the way home for 5% back on your groceries...

And if anyone actually does that, you get a 5% bonus for being awesome.