Final eye colour is reached around the same time as sexual maturity.
With Blacks I have found the bigger problem is leakage of black in the eye around the pupil is a bigger problem. Instead of being nice and round it tends to break up and extend into the rest of the eye.
I've seen this firsthand too. Chicken eye colour is fascinating really...and a lot more complex than I first thought. But then isn't everything genetic. lol
There is a good summary in one of my genetic books that says chicken eye colour consists of the following: blood vessels (red, orange) or the visible absence; black pigment on the front/backside of iris, or absence; opaque layer which causes pearl eye; carotene in iris (orange/yellow) or absence; genes which inhibit black pigment on one or both sides of the iris; genes which inhibit pigment distribution (barred, mottled).
It goes on to say there are more factors playing a role than meets the eye, (har har) and some even counteract the above. Nothing simple here, that's for sure.