I see an actual brown spot on the lens in this photo myself. That is not normal.
OP: Can you post another photo looking "straight on" to the eye? The angle of your picture is a little deceiving.
It could certainly be corpora nigra, but the angle of your photo makes it look like its on the cornea.
If it is not corpora nigra, I would have a vet look at it at some point. Since it has been there for a long time, it's not anything that needs to be seen urgently.