BlackRock, the enormous American asset manager with over $4.6 trillion of assets under management, has waged its first significant activist campaign around the G-Resources Group, a Hong Kong company that owned a gold mine. It may be Hong Kong, and it may be only one campaign, but companies should be fearful. That the words activism and BlackRock are mentioned together is deep with symbolism. BlackRock has historically shied away from openly challenging companies. Indeed, Laurence D. Fink, BlackRock’s chairman and chief executive, has become a thought leader in the pushback against what he terms “short-term” activism.