<p>I think you're right that the question at the bottom of your post ("What should be the conclusion regarding the effect of b on value?") is OK.  In light of that, I'm willing to reopen (and answer) it.  </p>

<p>For future reference, that question doesn't match the question in the title of your post very well ("why is a variable not significant on its own, but significant together with other variables").  Moreover, the title question is much more salient, and is a frequently asked question&mdash;that's why it was closed as a duplicate (rightly, in my view: I read it before adding additional duplicate targets and interpreted the question at the bottom in light of the title).  It is your comments here, and your edit to your post, that clarify what the real question is.  For the record, those clarifications are the way to productively deal with having your post closed as a duplicate if you disagree.  </p>
