If you are making a baked Mexican chicken casserole that uses black beans, what can you substitute for the blacks beans to enhance flavor but not totally change the dish into something else?
You have to ask yourself, what role to the beans play in this dish (see Are there any general principles of ingredient substitutions?)?
In this case, they are an (likely) an accent ingredient that provides some texture, and flavor, and some of the Mexican identity of the dish.
As an accent ingredient, you have a lot of discretion on how to change out the black beans. The nature of the flavor that they provide is not overly strong and aggressive, so this rules out ingredients with very strong flavors due to your desire not to change the nature of the dish very much.
Obviously, the most likely candidate is another bean, if your guests can and will eat them, as they will be most similar to black beans. Pintos are fairly typical of Mexican cooking.
Another likely substitution would be corn, which also adds texture and flavor, but this is already present in the recipe. If you can get it, you might try some hominy, but this is a more exotic ingredient (at least in some places) that might be surprising to your guests. It will provide an interesting textural and flavor contrast to regular kernel corn.
While not technically a substitution, you can also simply omit the beans. As an accent ingredient, their loss won't change the substantial outcome of the dish.
Spreading a little further out, a fresh legume like chopped green beans, peas or limas will work well with your other ingredients.
In the end, an enchilada is mostly a method (and technically the chili sauce that gives the dish its name) and you have nearly complete freedom to vary the ingredients according to your tastes and the availability of ingredients.