Personally, I think it's just a trait like blond or redhead in people. The blood left in the antler makes the most sense coming from a biologist and all, and I think some deer just tend to have more of it than others.
Having cut up numerous antlers, some dark and some light, I've found that the dark color goes far deeper than what I think tree sap could ever penetrate. For that matter, if you look at most deers antlers, very few are dark all over. Usually the tips and tines are lighter colored. THAT'S where most of the rubbing is done. Where the bases are rough and knobbly or on the back sides of the main beams is where they tend to be darker.
If you see a buck who's rack shape allows him to rub his rack smooth right down to the pedicle, it's usually lighter in color than the sides and back of the bases so how does rubbing darken the rack when the rubed areas are lighter? Yea, I know different kinds of trees... I'm just not convinced that sap can penetrate halfway to the center of solid bone.
It's probably a lot more complicated than anything we're guessing at. Kinda like 20 years ago when the big argument was whether bucks rubbed trees to remove the velvet from "itchy" antlers or to strengthen their necks. I remember some pretty knowledgable hunters back then arguing strongly for some pretty ridiculous reasons about that. Heck, they even argued over whether to call them rubs or scrapes.
Anyway, all I know is I LIKE dark racks. Big, gnarly, massive dark racks...... Who cares why they are dark or light. I certainly won't pass up a buck because his rack isn't the shade of brown I want.