I could be completely off, here, but I thought I'd share this and see if it applies at all.
My 6 year old, Stormy, is supposed to be a smoky (smokey?) black. His mom was a bay, no idea on Dad.
When I lived in Tennessee, he was pasture kept. His summer coat was horrible, a streaky brown that looked faded. He'd get considerably darker in the winter.
I moved to Phx, Arizona about 3 years ago. Since then, he is in considerably stronger sun, in constant turn out with a shade structure that he uses on and off. What changed dramatically was his diet - he was on a fescue pasture with a general supplement and alfalfa pellets there. Here, he's on alfalfa/ bermuda hay, the same supplement and pellets. He is nearly jet black all year round (taken today, although he's just been put back on a little Alfalfa after losing some poundage and looks just a touch browner than usual)
So, for my smoky black, sun doesn't seem to be the big diff. You may want to try some different feeds, etc. if you want your horse darker. I know that some people use Paprika to deepen the black, but that's not what I'm doing.