The Black Sand is the most interesting aspect to me, because if only ONE Of the liquids is turning blue in reaction to it...it's making me wonder if it's one of those special poisons that only becomes a poison if mixed with another component. Thus, by itself? Not poisonous. But if combined with its second deadly component.....

Of course, if the Black Sand is to blame, then that woulds certainly place a lot of suspicion on Yongnian.

...UNLESS his disappearance means that he's been poisoned too, but idk if that kind of twist would be introduced at this stage?!?!?