Yes, not uncommon. What you describe is a melanocytic nevus. A nevus is another name for a mole and they can also occur in the eye. In fact they can occur anywhere where pigment is present (skin, eye, inner ear, meninges, bone & heart). So it is not uncommon to have a mole in the eye. As with a skin mole, it should be checked. See an ophthalmologist periodically to ensure that it is benign & not changing.
Pigment in eye. Yes, there are focal pigmented areas on the retina of the eye in some people and even in the forward areas at times. There can be unusual pigmented lesions, some serious, that develop in the central nervous system as the pigment cells and nerve cells share some similar background during our development in the womb. Melanomas that rarely occur in the central nervous system are not caused by sun.