The open system theory is the foundation of black box theory. Both have focus on input and output flows, representing exchanges with the surroundings.
The black box is an abstraction representing a class of concrete open system which can be viewed solely in terms of its stimuli inputs and output reactions:
The constitution and structure of the box are altogether irrelevant to the approach under consideration, which is purely external or phenomenological. In other words, only the behavior of the system will be accounted for.
The understanding of a black box is based on the "explanatory principle", the hypothesis of a causal relation between the input and the output, and:
input and output being believed to be distinct,
having observable (and relatable) inputs and outputs,
being black to the observer (non-openable).