I have a roll of W&N white - it's from about 20+ years ago - but it does not have black dots.
I also have a roll of W&N "natural" - also from about 20+ years ago - which does contain brown flecks of the cotton plant. When I washed it before using it, it turned the water very yellow.
Both rolls have a scrim side made of polyester - that is almost like a light weight pellon - that the cotton fibers are needled into - and that is somewhat smoother than the "pure cotton" side.
https://warmcompany.com/frequently-asked-questions
The company does suggest having the scrim (smoother side) face the backing - but that the batting does not have really have a "right" or "wrong" side.