Small black spot on my gum. teeth x-rays came back normal. what could it be? have had it for about 3 weeks

Maybe amalgam tattoo: It could be an amalgam tattoo. They are fairly common. Amalgam fillings can turn the gum tissue gray or blackish. A quick visit to your dentist wouldn't hurt. They would be able to figure it out or refer you to a specialist if needed. Hope this helps!

Melanin pigmentation: No treatment as such is recommended or needed. Keep you routine dental check ups every 6 month.

How long ?: History of spot is critica. If it has been present forever, probably of no concern. If it is recent, it needs to be evaluated and possibly a biopsy.