At low power, you can detect an area of increased pigmentation in the lamina propria. Note that the overlying epidermis (or in this case of the hard palate, the squamous epithelium) is uninvolved. The increased pigmented is due a proliferation of spindled melanocytes containing melanin and surrounding melanophages.
At higher power, spindled melanocytes containing abundant melanin pigment is seen. It is not easy sometimes to distinguish melanocytes from melanophages (macrophages that have ingested extravasated melanin), but the cell with a round nucleus surrounded by abundant pigment (on the lower left) may be a macrophage.
Blue nevus is a dome-shaped nodule that has a striking dark-blue hue. These benign lesions occur due to arrested melanocytic migration. There is a female predilection (2:1), and generally occur on the hands, feet, buttocks, scalp and face. Note, however, that extracutaneous sites are not uncommon, including the cervix, prostate, palate and lymph nodes. These lesions are benign.1
Microscopically, the key features are a dermal (or in the case of the cervix and palate, lamina propria) proliferation of spindled melanocytes containing abundant granules of melanin. Melanophages will also be present. In the cellular variant, the melanocytes will be more closely packed together and less spindled (more plump).
1 McKee PH. Pathology of the Skin with Clinical Correlations 2nd Ed. London, UK: Mosby-Wolfe; 1996: 13.25-13.28.
2 Rapini RP.Practical Dermatopathology. Philadelphia, PA: Elsevier; 2005: 270-1.
3 Sternberg SS, ed. Diagnostic Surgical Pathology.4th Ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2004: 96.