The best or only way to identify authentic vs fake Yixing is to get to know existent known authentic clays. Therre are only so many existing authentic clays.
The next best rule is to estimate the age: anything made before 1995 are assumed authentic except proven otherwise. Vice versa, anything made after 1995 are assumed fake exept proven otherwise.
On the internet when they say they are Black Dotted Clay , they really means they are Black Dotted Clay Artificially Made with manganese .
It is still considered authentic because it was the clay of from the official 1st or 2nd factories even though it may has added chemicals (likely manganese) for the pigments. I have quite a few of this kind of clay and they are very good clays and all have very good permeability.
This clay was created by adding Cobalt Minerals, not Chemicals as commonly used today
mix of Purple and Green clays that exist naturally