One pernicious myth, perhaps just hubris, is that humans are the apex consciousness on our planet, even our galaxy. Never mind that we still do not understand consciousness, we believe, as children believe, we are the best at it. Wander through the Museum of Natural History to get the picture. Human evolution is the supreme act, the pinnacle story of life Earth, look there go the giant dinosaurs, inferior because they died out.
Call it the standard model. Sense a certain house-proud disregard for the other more primitive beasts. Witness our evolution, up from the apes. Dioramas of early human societies, displayed in the same manner as collected bison, and creatures from Africa and South America.
Astronomers have more or less proven, by means of mathematical probability, that other life forms not only exist, but actually are very prevalent in the universe, simply because the universe is so large and the abundance of other solar systems and planets so great.
It is also a mathematical certainty that life forms exist which surpass us in age, history, technological and intellectual might. They may not even be carbon or water based life forms! Those that are confident of the existence of extraterrestrial life know this in their guts. Others know it from first hand experience, claiming to have been contacted directly, or abducted. For Stanley Kubrick and Arthur Clarke in the late 1960's these were ground-breaking ideas.
What would contact with a higher level of consciousness feel like? How would we describe it? What would we see? Would the visitor leave signs, or a token that it had visited? I'll second what many before me have written and say the black monolith stands as a reminder left by a superior consciousness, that man has arrived at some stage of evolution, be it tool-making or space-travel . . . or life outside of time and space.
One might consider this notion another way. If the monolith is a sign of the passing presence of a superior intelligence, are we simply noticing it when we are ready to? In other words is the monolith put there for us to notice then, or now? On another axis, what evidence of superior life forms are we currently oblivious of, simply because our minds cannot recognize them as living?
And what if that alien consciousness does not live within three dimensions, like us, but rather exists in multiple dimensions? They would not be visible to us at all, unless, either intentionally or as a side effect of their presence, they left some sort of sign, like a crystal.
Hence that perfect black rock.