Yeah, it's a pretty weird design.Right after the first movie came out, I've seen some artists drew that blue part as being one with the shirt, so I went back and rewatched the whole movie just to observe her outfit and indeed it is never separate from the shirt.Also, if we look at official Equestria Girls dolls, we can see that Pinkie Pie wears a simple one piece shirt, while Sunset does in fact have a separate jacket element.But truth to be told, only way to know be sure is to ask to ask someone on the MLP staff, and I don't think anyone did yet. I certainly did not