Wiki melanonychia, correct ICD-9

703.8 is more appropriate to use if you do not know the cause of it. If the underlying cause is determined from a matrixectomy, code to that condition (i.e. melanoma, hematoma, etc). 703.9 is an unspecified code, avoid using that at all costs. 709.00 should only be used in reference to the skin and subcutaneous tissues, not the nail. Melanonychia is a black or brown pigmentation of the nail unit.

Check out the link below regarding code 703.8. I have found this website a good reference for looking at alternate terminology for codes:
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