Looks like a little blood blister/scab to me - maybe you dropped something on your toe, or stubbed it? I'm sure I've had similar thing once from shutting my finger in a drawer or something, didn't think anything of it, and it just grew out eventually. Nails are slow growing, so it takes time is all...
I didn't think there is such a thing as toe cancer... or nail cancer for that matter... and it's a pretty weird location for a melanoma I would say. The chances of it being anything sinister are miniscule to none.