I have a black spot under my third toe on my right foot. it's small but covers 1/4 of the toe. i'm wondering if i should be concerned about melanoma.

That's reasonable: I'd have that checked by a dermatologist, general surgeon, or plastic surgeon. Your concern is a reasonable one. Good luck.

ABCDEF: A-Age; 5-7 decade. African-Am, Asian-Am, Native-Am. B-Brown or black. Width greater than 3mm. C-Change D-Digit. Thumb or big toe most common. E-Extension of color change to skin. F-Positive family history. When in doubt, see a doctor.