I do get non-black shadows on my 5S with a stationary skylight and no precomputed lighting - then again I’m only using solid colours in my game (no textures) so it’s possible the skylight just lightens them!
Since I first tried a skylight many versions ago I have only been able to get it to actually affect the world properly by setting it to stationary - I imagine I’m doing something wrong, but basically any surface that is not in line of sight with my directional light is very dark with the skylight set to static. Even when I was using static baked lighting this was the case.
I’ve tried your plane method whilst allowing precomputed lighting + static lighting in the directional light (with a lightmap on the plane) yet all my shadows stay pure black.
Edit: I notice that deleting the default skybox gives me black shadows. Perhaps this is a factor?