I don’t know how wild of a blendshape you’re thinking of, but if you’re only using them for facial features/animation, then you don’t need to worry. Basic vector normals will hide your seams without an issue, no matter where they are… So long as you’re not blending where the cut is. However, if you’re doing something such as making a skinny guy fat, then you will definitely get a seam.
One of the biggest issues in doing a modular system with blendshapes that alter the body is that the UE blendshapes don’t have vector normals. If they did, then there wouldn’t be a problem. We could modify the vector normals for the blends and no one would be the wiser. As it stands, you basically have two choices: Live with the seams, and do your best to hide them when the blendshapes are applied, or expand your skin mesh to include every area visible for all outfits.
TL;DR: Blends for the face are fine with a modular system, so long as you don’t cut the model along an area that is being modified.