It's not blackmail by either the legal or common-usage definition of blackmail. Even if Uber followed through with such a sleazy plan, it would not be blackmail. For blackmail, a demand must be made, accompanied by a menace. Floating an idea that you plan on digging up dirt? That might have been an incipient but ill-advised plan, it might be idle dreaming, or it might be it might be machiavellian FUD.