The cube will be lightly obscured for characters with darkvision and no light sources.
Looking at the rules for Vision and Light:
In darkness without a light source, a character without darkvision can't see the cube:
A heavily obscured area—such as darkness, opaque fog, or dense foliage—blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in that area.
In darkness without a light source, a character with darkvision sees as if in dim light, meaning they have disadvantage on sight-based perception checks:
Within a specified range, a creature with darkvision can see in dim light as if it were bright light and in darkness as if it were dim light, so areas of darkness are only lightly obscured as far as that creature is concerned.
In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.
In darkness with a light source, a character conducts perception checks to see the cube normally if it is in the area of bright light cast by the light source.
Bright light lets most creatures see normally. Even gloomy days provide bright light, as do torches, lanterns, fires, and other sources of illumination within a specific radius.
Nothing in the Cube's Transparent trait indicates that darkvision provides any special disadvantage in spotting the Cube. Relative to a group of characters with light sources, characters with darkvision will have a harder time seeing the Cube, but they will have a harder time seeing anything else as well.