Delicate, brown or black pigmented non cariogenic plaque found on the enamel at the cervical margin of the tooth is:

A delicate pigmented dental plaque, called the mesenteric line by some, was described by Pickerill to be a plaque of brown or black dots that coalesce to form a thin, dark line on the enamel at the cervical margin of the tooth. Pickerill, F Bibby and Shourie noted that the presence of this line is often associated with a relative freedom from dental caries. Ref: Shafer's textbook of oral pathology 7th edition page 386