Hi Chase, yes curves can only be filleted if the 2 curve segments that touch at a sharp point are coplanar.

re:
> Or should I find a workaround such as trimming back the segments near the desired fillet, and using the blend tool?

Yes that would work.

But are you trying to get something like this, here using Boolean difference with a 2D curve as the cutting object, then filleting the solids:







- Michael