These blueish cacti have blue-berry like fruit. And I can tell you here and now, they are delicious. If you are ever trapped in a desert-like environment, in Mexico, and you must find some food, I would try these. Now while it is true that tunas, i.e. prickly pear fruit, are even more delicious, unfortunately they are covered in spines, whereas these are not.
The name Whortleberry is also common to a type of berry-bearing heath from Europe, as well as a particular type of American blueberry. So much to learn.