It is important to advise clients with cupping marks to avoid direct sunlight on the marks and to use sunblock or clothing to cover them when outdoors. Exposure to sunlight may make the marks last longer, especially in darker-skinned individuals.
The article on Postinflammatory Hyperpigmentation is from the Journal of Clinical and Aesthetic Dermatology and can be found here:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2921758/