Just to be very clear, this question is very much related to, but distinct from a previous question I asked a few years ago: Does Darkness cast a shadow? - I still am not quite satisfied with SevenSidedDie's very excellent answer (they usually are, aren't they?). This question asks does light penetrate through the darkness spell or does darkness cast a shadow. And it is distinct from Does the Darkness spell block vision?, which asks if the darkness blocks vision through it.
Is the Darkness spell visible from the outside? To make the question more clear, if darkness were cast on the surface of a flat plane, would I see a spot of darkness on the ground, or would I see a hemisphere above the ground?