The only way to do things like elbows are to do them in the standard bind pose, dupicate the model and do a sdk for the blend shape eg: its driven by the elbow joint, then pose the skinned elbow, then work on the target. Its best to use a split GUI layout so you can see the changes on the skinned model as you make them.

Also be aware you may have to rearrange the inputs order depend on where the skin cluster is etc after doing a blend shape otherwise you'll get double transformations

cheers
Jay