Same night we got a second Black-Purple Eviscerating off of Terror From Beyond on Hard Mode. This is our second full clear on HM (cleared it last week as well) and no crystals dropped from Kephess or TFB at all last week. It could be that crystals can drop randomly from any of the bosses, but we've only seen them on Kephess and TFB.
If we see anything other than Black-Purple in the future I'll update this thread and let you guys know.
We got more Black-Purple in the 2 weeks we being in HM TFB than the all the months we spent farming HM EC. Seen Black Purple drops off the two hidden chest, off the 2nd boss etc. Any boss + hidden chest have a chance to drop it.