probably a cataract....go see an optometrist.
I also have a very tiny black spot but it is in my left eye close to the pupils. It is so tiny that it barely bothers me. I have never gone to an eye doctor for it because it does not affect my eye and it has not grown in size. I am not to sure what it is. However if the dark spot is affecting your eyesight then it time to visit the doctor.
go to get your eyes tested and see a doctor i have had it for ages my tests all came back clear,so they put it down to the nerves in my neck do you do any exercise or lift anything heavy?