6- What is the black wax mouthpiece found on some yidaki
The dark-coloured wax on the mouthpiece of some yidaki is the wax from the native Australian Trigona stingless bee, this wax is commonly called "sugarbag beeswax". The colour may vary from brown to black depending on the species of Trigona bee and the quality of the wax and can be quite difficult to find. Modern didgeridoos typically have a yellow-coloured beeswax which comes from the hives of the common European honeybee.
7-Why do some yidaki have no wax on the mouthpieces
Some yidaki have no wax whatsoever on the mouthpiece for the reason that none is needed. For Yolngu craftsmen and musicians, an ideal mouthpiece opening is anywhere from 28-33 mm. If an instrument has these dimensions then one can play the didgeridoo with a natural mouthpiece if so desired.
If the opening is bigger than this, a rim of sugarbag beeswax may be used to reduce its diameter. If it is smaller than this, the craftman or player may open it wider with a pocket knife or any edged tool. If the natural mouthpiece opening is already of an ideal size, it is left alone and no further work is done to it.
You may of course add beeswax to your yidaki if you prefer the comfort of beeswax and the cushioning effect it gives the lips but it is not essential