In discussing this question, I came to realize that the real question was whether Darkness blocks vision or merely creates “darkness”.
Per the wording of the spell:
Magical darkness spreads from a point you choose within range to fill a 15-foot-radius sphere for the duration. The darkness spreads around corners. A creature with darkvision can’t see through this darkness, and nonmagical light can’t illuminate it. (emphasis mine)
Normal darkness is defined in the game as creating a heavily obscured area and the only description added to the darkness is that it is magical, which (to me) just means that it is created by magic and is subject to the magical rules.
Darkvision is defined in the game as basically being able to see in darkness as if they were seeing in dim light. So, the text “darkvision can’t see through this darkness” merely means that it affects darkvision in the same way it affects normal vision. (See earlier versions of the spell below, which had similar wordings.)
I see nothing there that implies it is a barrier to vision, just an active and utter absence of illumination.
Previous editions have varying descriptions. What's the history of the Darkness spell? and this wiki article give accurate accountings of the various versions.
The 1e Wizard version seems to actually imply a sphere of opaque blackness that even blocks infra/ultravision, while the 1e Cleric version is a reversal of the Light spell and creates totally normal darkness, with no block to special visions.
The 2e version is similar to the 1e Cleric spell. It creates an area of darkness “equal to an unlit interior room”.
The 3.0e version comes closest to the 5e version. It “causes an object to radiate darkness out to a 20-foot radius. Not even creatures who can normally see in the dark (such as with darkvision) can see in an area shrouded in magical darkness.” Still no reference to any sort of opacity and the wording makes it clear that darkvision is being treated specifically here.
3.5e Darkness is similar, although instead of darkness, it creates “shadowy illumination”. This affects darkvision as well as normal vision.
4e doesn’t have a Darkness spell, but rather a Cleric Utility called Veil of Darkness, which creates “a zone that is heavily obscured and blocks line of sight.” So, finally, a reference to opacity, in “blocks line of sight”, although I question if opacity was the intent.