.blend format doesn’t contain any vertices or faces or bones. It’s just a meta data that used by Blender to construct scenes. So you can’t just import Blender file, because there is nothing to import. You’d literally need chunks of Blender itself to be in the UE4 plugin, so that it could generate your scene from .blend file (that’s my understanding of what was explained a while back by Blender devs when someone was asking about importing .blend files). And that is the issue. If .blend file had vertices, bones, etc., there would be no issues or license incompatibilities.