This is an interesting post. I thought that black pandit hats only existed in Nakthang tangkhas!
Nonetheless, I've read from somewhere that whenever a Lama or a presiding Acharya does pujas for any of the four enlightening conduct, namely; pacifying, increasing, controlling and subjugating, the lama will wear regalia of the relevant colors connected to these activities. For example, a Lama doing healing/pacifying puja would be donned in white robes, white crown and hold implements concerning this mode of conduct.
Hence, looking at this pic, this Lama is conducting a Mahakala Puja which is a form of subjugation puja. We can see also that in the fire pit there is a triangular shape mandala with tormas and offerings inside. We are able to see a black cloth on the side which is his ritual implement for subjugation.
I am unsure if this hat is valid in accordance with our lineage, but I think what I just posted here would be of the closest explanation I can give. Hope this helps