I just noticed a little black spot on my very back tooth on the outer side next to my cheek, it seems like there's a very small pit there but cannot be sure since the point of the toothpick sometimes hit it and sometimes didn't seem to hit anything. Is there a chance whatever it is will fill in on its own? No pain involved.

Watch,

It could be stain. If there is a bit of a notch at the gumline you have an abfraction which is usually caused by clenching and grinding OR you've worn it by scrubbing with your toothbrush. If there is no sensitivity to cold or sweet it is probably not a cavity.

Can stains have slight pits? And would cold tap water trigger the sensitivity?

The tooth can be pitted where the stain is. It may be weakened enamel (decalcified)which could feel pitted or rough. Yes, cold tap water could trigger the sensitivity.

I would go see a dentist. A pit with sensitivity sounds like a cavity to me. Take it from someone who has a mouth full of cavities right now it is better to get them filled when they are small. When was the last time you saw a dentist?