Rex Kerr's answer is correct, but only part of the story. If the cube is expected to stay black for any length of time, it must incorporate some (mumble/handwavium/nanotechnology). The problem of dust has to be dealt with.
Outside of clean rooms, there is always a fairly remarkable amount of dust floating around, much of it silicon dioxide.* This will get trapped on the surface and become visible, degrading the apparent blackness. Apparently, the cube needs active nanotechnology to either swallow dust particles or repel them. Each has its problems. If the cube swallows the particles, what does it do with them? Discretely spit out the day's accumulation when it thinks no one is looking? And if it repels them, how? A sort of cheat might be to claim that only the sides remain dust-free, which would take much less energy/handwaving, while dust which accumulates on the flat top is simply not visible from the ground. If the entire cube is to remain black, the cube must either ingest dust or transport it to the edges and dump it over the side. This will require some very entertaining nanotech.
- All gems will accumumulate dust during wear, and other than corundum-based (sapphire, ruby), topaz or diamond, with emerald a borderline case, any contact will cause microscopic scratches of the gem's surface, dulling it. Even the hard gems require occasional repolishing if they are worn often.
If the cube lands near Mecca and is slightly cool to the touch AT ALL TIMES, you have a problem. Summer near Mecca is hardly temperate, and the cube is presumably found outdoors, exposed to the sun. To feel cool, regardless of physical temperature, the cube (at least the surface) must have high thermal conductivity. A very high thermal capacity would help, too. This will mitigate the temperature rise during the day and facilitate energy release at night. So it's possible that the "slightly cool" label was applied when the cube was discovered, and was due to the contrast between the constant temperature and the hot surroundings, but it feels warm at night when it is warmer than the surroundings.