Why is copper II oxide black?

The blue colour of the other copper compounds that you have mentioned is not specifically down to the copper compound itself but to complexes that it forms.

For example, copper sulphate is often quoted as being a blue compound, but copper sulphate is actually colourless - it is the complex with water ligands (copper sulphate pentahydrate) that is blue.

The reason for the colour is that the presence of the ligands causes an interaction between their electron clouds and the the d-orbitals of the copper ions. This results in the d orbitals (which normally would all be of equal energy) splitting, which allows certain electrons to be promoted from one part of the split d orbitals to another, and when they return back to the lower level, they give off energy which happens to be in the visible region of the spectrum. Therefore we see a blue colour.

The structure of copper (ii) oxide does not favour ligation and the formation of complexes.

Copper Ii Oxide Color

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RE:

Why is copper II oxide black?

Copper is a transition metal, and copper II ions are blue.

Other copper II compounds have the blue colour, (copper II chloride, carbonate, hydroxide) so why not copper II oxide?

Copper is a transition metal. Transition metals will form coloured compounds.

Anhydrous copper (II) carbonate is actually green (not blue as you referred to), while copper (II) oxide is black.

CuO can be obtained from the decomposition of CuCO3: CuCO3 --> CuO + CO2

To the best of my knowledge there is really no explanation for the colour of CuO. The only simple answer is that transition metal will form coloured compounds, so in this case CuO is black.

There are many other transition metal compounds which are coloured.

For example, chromium(III) chloride is violet -- http://en.wikipedia.org/wiki/Chromium(III)_chlorid...

Silver iodide is bright yellow -- http://en.wikipedia.org/wiki/Silver_iodide