Histologic study of facial skin biopsy specimens from black and white subjects disclosed why blackheads are pigmented only at the tip.
In normal skin, dopa-positive melanocytes were found only in the superficial infundibular region of sebaceous follicles. In closed comedones, the melanocytes were in the same superficial location and they produced only a tiny amount of microscopically visible melanin. In open comedones, numerous, large, enzymatically active, melanocytes were present in the uppermost portion, disappearing completely one third of the way down.
Because of the superficial location of melanocytes, only keratinocytes near the orifice can acquire melanin.
Kaidbey KH, Kligman AM. Pigmentation in Comedones. Arch Dermatol. 1974;109(1):60-62. doi:10.1001/archderm.1974.01630010040009