The move, announced Wednesday, comes after BlackBerry (TSX:BB) shares had a strong start to the year, gaining more than 15 per cent since the end of 2013.
BlackBerry stock ended six cents higher on Wednesday, closing at $9.20 on the Toronto Stock Exchange.
Fairfax, which is the largest shareholder in BlackBerry, led an investment group that injected $1 billion into the company last fall.
The insurance and investment firm invested $250 million as part of that initial deal which gave the investor group an option to make an extra $250 million investment before a Monday deadline.
Fairfax said the additional BlackBerry debt was being bought for investment purposes.
Fairfax president Paul Rivett told The Associated Press his firm believes the market is only just beginning to see the true potential of BlackBerry under new CEO John Chen and the team he has assembled.
The debentures pay six per cent interest and are convertible into BlackBerry shares at a price of $10 per share.
Over the past month, BlackBerry's new chief executive John Chen has been speaking to the media and making the rounds at various industry events to trumpet plans for the Waterloo, Ont.-based company's future.
His plans include a five-year agreement with Foxconn where the Chinese manufacturer will produce smartphones for the company, including a model for Indonesia early this year.
Chen also wants to focus on keyboard smartphone models after sales of its touchscreen BlackBerry Z10 flopped.
Fairfax head Prem Watsa has been one of BlackBerry's strongest supporters in recent months, leading the charge to drum up financial support and playing a role in the hire of Chen.
Before leading the $1-billion investment in BlackBerry, Fairfax had contemplated making a takeover offer for the company to take it private.
Other investors in the initial investment agreement included Canso Investment Counsel Ltd., which put in $300 million, while funds managed by Mackenzie Financial invested $200 million.
Another $100 million came from Qatar's sovereign wealth fund, $70 million from Virginia-based Markel Corp., $70 million from funds managed by Manulife Financial Corp. (TSX:MFC) and $10 million from Brookfield Asset Management of Toronto (TSX:BAM.A), according to securities filings.
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