I'm joining this late, but it could be either a doublet or a dyed Mexican matrix opal, both of which could be mistaken for black opal. There are doublets made with Mexican crystal opal on top and either black onyx, obsidian or other black material (plastic for one) on the back. There are also little mosaics made with black plastic on the back and opal set into clear resin on the top.
The other possibility, Mexican matrix opal, is very similar in appearance to treated matrix from Andamooka, South Australia. The Mexican matrix is made from a hydrophane opal which is porous and light weight and in the traditional method this was smoked--actually wrapped in paper and then burned, with the smoke darkening the stone.
You may have one of these. However, there actually IS precious black opal from Mexico, a very rare item. It is unlikely that you have one. If you did, it would look very much like a fine Mexican opal but with a black background color instead of the usual clear, yellow, orange or red.
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