So, here’s how the model looks in Blender:
Perfectly normal normals!
Now here’s how it looks in Unity:
Terrible!
Why are the normals okay in Blender, but when I bring them into Unity (as a .Blend file) they get screwed up?
How can I fix this?
Apply the Scale, your mesh is likely scaled negative along an axis causing the normals to be reverse expectation.
Have you tried recalculating your normals in Blender and re-exporting?
In Blender, select your mesh and enter edit mode. Once in edit mode, in the Mesh Display section circled in the image, select the box for display face normals as lines.
This will show the blue lines you can see on each face of the mesh, like on the cube. The lines are only on the outside of the normal and point away from it. If any of the exterior faces of your mesh do not have the blue lines, then you need to flip them. You can do the whole mesh at once by selecting the entire mesh and pressing CTRL-N. Alternatively you can select an individual face and go to Mesh>Normals>Flip Normals.
If this doesn’t work, maybe try exporting your mesh from within Blender as an FBX rather than importing the .Blend file directly into Unity.