Wrong vertex colors imported from blender

Uhm when you have a problem in Unity it would make more sense to post a screenshot from Unity. Maybe your imported mesh has two submeshes and the renderer uses two seperate materials. If this is the case you probably only use a shader that uses vertex colors in the first material but not the second. Check the materials array in the inspector.

Other than that we can't really tell what might went wrong in Unity. You haven't even mentioned what model format you used to export / import the model. All those things could be seen from a screenshot of your object selected in Unity.