I was doing this boundary boss for a connecting rod and was trying to join section AA(In image) to larger side of the rod but failed to do so either with Boundary boss or with Lofted boss and guide curves.
Because your sketches do not tel to the program an easy way ! :) So you cannot loft (blend) a 3d sketch with another sketch! You need two 2d sketches or of some entities on two plans.
One thing few people know... be sure that the two sketches have the same number of points. See picture and SW2015 part.
The solution often seems easier after she was found!
I think the drawing man make an error. The designer must know that a craftsman can make what the machine can not always do. Software like Solidworks does not know everything, but it can do everything that the machines do!
So I changed to a minimum the problem by adding an element without impact but provides a logical solution.