Biopsy is in order to evaluate for possible oral melanoma. The mouth is a rare location for melanoma and accounts for < 1% of this cancer. While cutaneous melanomas are linked to sun exposure, risk factors for mucosal melanomas are unknown.
Source
Collins, B. DDS. "Oral Malignant Melanoma" eMedicine. 21 March 2008.
Image source: http://dermatology.cdlib.org/141/case_presentations/oralmm/1.jpg