In a large bowl, combine black beans, corn, salsa, tomatoes, and balsamic vinegar. Refrigerate until ready to serve (best made 1 to 2 days ahead of time before serving).
Add diced avocado to mixture just before serving.
Place mixture into the hollow of an endive leaf and serve.
* If using canned corn kernels, drain before using in the salad.
** Diced fresh tomatoes may be substituted. Add some oregano, garlic, and basil.
Betsy's Black Bean Salad Recipe: https://whatscookingamerica.net/EllenEaston/BetsyBlackBeanSalad.htm