I'm wondering if there's a more subtle relation. Moreover, we KNOW there's a deep background here, what with Shallan's hair going back to Horneater ancestry, no?
What I'm thinking is that it is possible to have a lot of singer sDNA without being visibly singer-like, so to speak. In Kaladin's case, this explains how easily he heals towards his Spiritual ideal of having the brand. Because I don't think that level of Cognitive influence is normal. I'm not saying Kaladin is like Lift, or maybe I am, but I am at least saying that maybe the sDNA component of the darkeyes manifests outwardly just as their dark eyes (and the eye-lightening is a comportment towards an idea of lighteyes being more purely "human"), but otherwise involves a significant ancestral relationship with the singers (assuming, as noted, that we're not talking about an engineered crossover via Cultivation/the Nightwatcher, although that description seems "wrong" to me).
Indeed, maybe it wasn't even possible for humans right out of the "Tranquiline Halls" to bond with spren? Maybe they HAD to admix with the singers, over time. I wonder how actually strict the hyper-ethnic distinction really is, here, by now.
We've seen that marriages can be highly Invested affairs on Roshar, I think. Or, at least Dalinar and Navani's was, but this is a proof by example. We also know that Honor and Cultivation were romantically united. So I daresay, early human-singer marriages might have been routes for a major sDNA transfer between the populations. So, some of the transfer is normal genetics, some is Invested genetics. Some crossovers are Physically emphasized (Herdazians, Horneaters, who else?) while others are more Spiritually manifest (darkeyes and lighteyes?). Traces of gemheart biochemistry allow spren-bonding among the neo-humans.
Although, I would have to also figure out how the Iriali fit into this scheme... among others...