Overlapping AoE effects work.
It's just that Darkness and Cloudkill do not. For whatever reason, Darkness will cancel Cloudkill and prevent it from being cast in the area.
Ironically, Cloudkill and Hunger of Hadar works which is basically the same thing as Darkness.
As far as removing it, it's considered "Magical Darkness" so only Radiance of Dawn will remove it (As well as Gust of Wind which removes clouds) while things like Daylight do not (These only remove regular "Darkness" not "Magical Darkness")
