You can bake normal maps from geometry in Blender. Unity works very well with normal maps. Follow the instructions here, but instead of baking a displacement map, bake a normal map. How to make a displacement map from existing 3d geometry
Displacement maps work great on models with high levels of geometry. They work by displacing the models vertices based on the texture value. dense geometry does not work great in video games, because that means more faces that the computer has to render 60 times per second (usually). Instead a normal map represents lighting information for each pixel within the texture. Without deforming the actual geometry, the texture itself can receive detailed lighting information, creating the illusion of depth.