hmm.. not necessarily all the time though. it especially happens to duan or benshan, and a well fired duan-ni, just a nice yellow before it goes orange, is a state where it wouldnt really tu-hei. but if on the lower firing side.. it happens when the pot meets tea! i think of it to be some reduction going on with reaction to the tea over time. it seems that the firing of duan/ben is quite a narrow margin and the temp gradient in the kiln may seem to have certain effects, together with the neutral state of the ware (cyclic oxi/redox in d.kiln), it seems that many many many many many of the ROC and earlier Duan/Bens end up vomitting.AT333 wrote: 吐黑 happen often to those pots dug from cemetery. The spooky black verticle lines often appear when the pot is use to brew tea. It is hard to rub off but there ways to do it. I am curious if the ben shan lini 吐黑 is the same or different type of vomit
on the other hand, excavated pots are not necessarily low fired, but instead if it is buried somewhere water logged, it will 退窑 (recede fire) over time, turning the zhuni or hong ni into something that resembles shantou ni texture.. but if buried in appropriate "containers" or appropriate locations where water drains away, the pot that remains dry will come out quite decent. thats perhaps why some of the early/mid qing pieces are so expensive and so beautiful, whilst others from the same era, despite being old, have receded firing and that affects its tea potential and overall aesthetic etc.
in principle, possible to get rid of the tu hei spots if one reacts the pot chemically with something.. but thats a different issue altogether
lets see if i can find something in my upcoming trip to taiwan again~
could affect the aroma coming up from a yixing after pouring the water in. But as soon as there is enough water, new water would fall on more water rather than the porous bottom of the pot. So it might only have an influence towards the beginning of the pour 
