Here are larger pics I could find (mainly through Paris Match's 'Royal Blog'), which give us a good view of the brooch.
I am not a specialist of opals, so I will not give any opinion whether all opals are black opals or not. But the different shades of the opals are very beautiful and make a very nice effect.
And two close-ups - a bit blurry, sorry, but that's the best I could get!