My custom detail meshes appears black

Hi! I've been playing around with Unity for a while, but this is my first post ever to this forum. So I hope this question are in the right place:

I'm trying to add custom detail meshes, I add them, I set the rendering mode to VertexLit instead of grass. But when I paint them onto the terrain they appear black, pitch black. The objects are in the fbx format. What could be wrong?

best regards!