Assuming this is the case, here’s the documentation on UV mapping for Blender if you are unsure how to do it https://www.blender.org/manual/editors/uv_image/uv_editing/unwrapping.html
Note that the only method of texture mapping that UE4 will accept is UV: Generated, Reflection, Normal, Stress, Tangent etc. will be ignored and treated as unmapped, hence the flat colour.