Why does Unity show dark regions on a model with animation created using Blender?

I sculpted a model in blender, and I want to import that to Unity. When I import just the model(without animation), Unity shows as is. However, when I attach armature to the body and automatically assign weights to it with the intention of creating an animation, Unity import shows dark regions all over the model. All the face normals look fine. I am not sure what the issue is.

Model in Blender: Model in Blender:

Weights: Weights

Dark patches in Unity: Dark patches in Unity