It is indeed an unofficial 'Official' of Tibet.
It's interesting, and suggestive, to note that Jammu & Kashmir, which was also in the Himalayan region abutting western Tibet, and which had territorial claims over the region (and which in the western region of Kashmir - around Leh - was Tibetan in language and culture) also used black for its Official stamps:

although the British had the Jammu & Kashmir State PO closed down in 1894.