A friend brought up an interesting question to me- if hypo dragons are lacking certain pigment, are they able to blackbeard? What about white dragons? What exactly causes the black in the beard?
Hypo is a reduction of pigment, not a lack of pigment. The color changing may be a mottled black or greyish rather than a true black beard any wild type morph can produce.
I don't know if a bearded dragon with Leucism could black beard. That's like albinism, not a true absence of pigment but a severe reduction. I'm not familiar enough with reptiles to answer that, which is the answer you're looking for.