It seems pretty clear to me. Somewhat ironically it's actually both. The spell description specifies an area of magical darkness (read: absence of light) that cannot be seen through by mundane sight or darkvision (therefore appearing perfectly opaque to creatures using those senses).
You could argue whether the area is a perfect geomatrical object or is more akin to fog with frayed edges I suppose, but apart from that I see no issue with its description.
