This occurs in 4.10 and the 4.11 Preview 6.
For some reason converting a BSP to a static mesh produces a mesh with incorrect lightmap settings.
After lighting is built, the mesh appears black or very dark in the level.
After some testing changed the lightmap coordinate index in the static mesh settings from 0 to 1 which fixed the issue.
It was really not easy to find this out, and for any uninformed user it would likely be a nightmare to fix as trying to research the issue led me to no solutions.
The problem is easily reproducible in a new project:
New level, add BSP, convert to static mesh, build lighting = black object.
Open mesh and set lightmap coordinate index to 1 = properly lit object.