Or could it be something else?
Mind you, I haven’t yet created its textures yet, nor a UE4 material.
Click on it - in the details panel you should see the material - open it - check the colour that you use. Does this happen after the light rebuild? 
Make sure it is receiving some sort of lighting, direct or indirect. Then build the lighting. The default material if you haven’t assigned one should be the gray checker pattern. If it is a metal material, it needs a reflection capture somewhere in there to provide specular reflections.
, I failed to mention that for now, the whole house (including the door), is the same mesh. I already know that I’ll separate that door on its own eventually, so maybe that material behaviour will go away then.
But yes, that black material appears only after I rebuild the light.
StephaBon, as for receiving light, what about the walls just next to the door? They do receive light just fine, shouldn’t the door too? Or, is this the way the light behaves in UE4?
I am not worrying too much for now as I know I still have to create the texture for the UE4 material along with all its settings.
DPC
Mmm, this is odd. I added a Directional Light, a few meters away from the door, the black went away right away! However, as soon as it was done rebuilding the light it came back 
DPC
Sounds like something might be wrong with the lightmaps if it’s fine with a dynamic light but black with baked lighting.
Make sure that you have a proper 2nd uv channel with a lightmap -> ://