but there's a scene in episode 13 (18 minutes in - the scene in my icon!) where Zheng Yi tries to use her power against Zhu Jiu and Ya Qing, and we see them both shielding themselves against it with black energy. But Da Qing, also Yashou, is susceptible.

Aha, great data points! Thanks! :-)

(You'd think I'd remember that, having made made that icon, but no...)

ETA: My theory is that Dixingren and Yashou with sufficient raw power can use it directly for all kinds of things, including protecting themselves, without using a specific power.

*nodnod* That would explain Zhu Jiu's various skills. (I'm never sure how his mind controlling Zhao Yunlan in the lab is supposed to work. ZYL's hypnotised by Zheng Yi's power, and then Zhu Jiu's ball of energy adds specific instructions? Which presumably ZYL's aware enough of that he knows what to do, even if he's not compelled?)