A tentative takeover bid for BlackBerry from its largest shareholder collapsed yesterday, clouding the immediate future of the company, the New York Times DealBook blog reported today. Fairfax Financial Holdings of Toronto, the insurance and investment company, had made a conditional, nonbinding offer to buy the 90 percent of BlackBerry shares it does not own for $9 each, valuing the company at about $4.7 billion. But Fairfax announced yesterday that it would not proceed with the offer, setting the once-mighty company on a path into the unknown and sending its stock into a tailspin. Shares of BlackBerry fell more than 16 percent, to $6.49. Adding to the uncertainty and confusion was the company’s announcement that Thorsten Heins, who was appointed chief executive in January 2012 with a mandate to turn the company around, had stepped down. BlackBerry said that John S. Chen, the former chief executive of Sybase, would become BlackBerry’s executive chairman and acting chief executive but offered no indication of how long he would serve in the top management role. In place of a takeover, Fairfax and a group of institutional investors will invest $1 billion through debt securities that can be converted into common shares at $10 a share. Exactly how BlackBerry will use that cash to restore its position in the smartphone market is unclear, and neither Fairfax nor BlackBerry would identify the other investors.