BlackRock Inc. (NYSE:BLK) shed some 760,000 shares of its stake in Boston Scientific Corp. (NYSE:BSX) a day before the medical device giant is set to release its annual earnings report to Wall Street.

BlackRock, which controls more than $3.2 trillion in assets, assumed a stake in Natick, Mass.-based Boston Scientific following its purchase of Barclay’s Global Investors last December.

According to stock ownership forms filed with the federal Securities & Exchange Commission, BlackRock owned some 76 million shares of BSX stock after the Barclay’s buyout, good for a 5.2 percent piece of BSX, making it the firms fourth-largest shareholder.

But BlackRock filed an amended form Feb. 9, disclosing that its stake is now 767,000 shares lighter. The asset management company owns less than 5 percent of the firm, according to the filing.

All eyes will likely be on Boston Scientific tomorrow as it looks to report its year-end results for 2009. Wall Street will be watching closely to see if BSX picked up any market share for its cardiac rhythm management business, especially in light of the relative softness of competitor St. Jude Medical’s (NYSE:STJ) sales of pacemakers and defibrillators in 2009.

Fourth-quarter CRM sales at St. Jude slipped one percent and annual sales grew three percent, mostly from international sales; U.S. sales were flat.

A group of analysts polled by Thomson Reuters expects Boston Scientific to report earnings of 13 cents per share on sales of $2 billion for the fourth quarter.