You should not need to weight paint at all, as long as the blade is parented to a bone it should be ok. Meshes parented to a bone move with that bone for those not familiar with rigging. Weight painting is generally only needed when you want something to deform (like a character).
I am kinda confused as to why weight painting helped you get stuff to work, unless this is a blender to unreal issue. It might be just that now that I think about it. Maybe you have to weight paint if you are coming from blender? I know absolutely zero about blender’s rigging system. I tried using it and it was completely different than maya.
@Starkium are you pretty familiar with blender rigging? Can you not parent in blender like you can in maya? That would explain your initial issue.