Realistic understanding claims that the state is the basic actor in international system. Realism only care politics, does not care decision making process, domestic politic system. As a result, realism analises the state as a black box.
The black box theory in realism means that structural realism understanding does not analyse domestic policy. In this sense, some questions are not important for the theory. who is manage the state? What kind of regime? What kind of view do government or regime have? They suppose that there is no relations between domestic policy and foreign politics . There are some domestic factor about policy as distinct from decision making process. The social, cultural factors.
In the realist understanding, international system has a anarchical structure. All of states compete for the power in the world. There is no authority over the structure of the state in international arena, so the power race contunies in process. Talking about domestic policy is not correct in this sense, because there is high authority in the state structure.
When Syria is analised as an example from black box theory in realist sense, and when internal dynamics are ignored, to understand events in Syria is not possible, because the Arabia Spring is not enough to explain events in Syria.
Classical realism was against parallel structure, and state was basic actor in international system, so they do not care domestic policy, because domestic policy approach and external policy are not same, in external policy there is no high authority, but domestic policy has a authority. (government) In addition, there is a difference between neorelism and classical realism. It can be institutionalism, neorealism explains to state as a basic actor, like classical realist approach, but neorealism splits up from realism about institutionalism.
To sum up, not to care domestic policy is not rational approach , because human beings are value (moral) creature, so to break off the relations is not possible with social, cultural life .