This is a classical discussion. The correct term actually depends on how you are describing your system - are you using a light visibility metaphor or a mathematical/CS concept? Since the mechanims do not show all details of its execution, you can say that the mechanism provides an "opaque" view of the behavior, because the user does not see how it works. This is actually the opposite of transparent - as you point out in your question. However, it is not that simple. This expression comes from a mathematical concept, the referential transparency. From Wikipedia: " An expression is said to be referentially transparent if it can be replaced with its value without changing the behavior of a program (in other words, yielding a program that has the same effects and output on the same input)." There is also the derived concept of transparency in HCI: "Any change in a computing system, such as new feature or new component, is transparent if the system after change adheres to previous external interface as much as possible while changing its internal behaviour."
So, yes, you can use the transparency concept to descript your system's behavior abstraction.