Hi Negaunee,
I would think of the different software as a one-way pipeline rather than something you can freely move assets between. You’ll find the least resistance this way:
Blender for creating 3D models → Mixer for Materials/Texturing → UE4 for rendering/game-play
You can certainly export models from UE4 using FBX and import those into Blender, but the results are messy. I’d only do this to edit geometry though, not materials.