No, "Shadow" itself is not a traditional element, per say. But, in the ninja world it would have to fall under some type of affinity, if not a combination of two, like many others.

You also forget that Kishi just introduced the elements a few chapters ago, and is basically pulling this stuff out of his ass as he goes.

Kankurou is a Jounin, but we have only seen him use puppets. Are puppets an element? We have only seen Temari use wind so far, and Neji... who knows what element Byakuugan is using, but we haven't seen two from him either.

Kishi is making this stuff up, and at times contradicting himself. So don't be surprised if he decides that Shadow is a combination of two elements, just for the purpose of promoting Shika.

Another thing to keep in mind is the translation. Things always translate differently depending on who you ask. That particular translation above says two elements are required to be a jounin. But you might find another who translates it more along the lines of "Only jounin level ninjas have two elements" and not necessarily saying its a requirement.