BlackRock Buys State Street Research  NEW YORK (Reuters) - BlackRock Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=BLK.N target=/stocks/quickinfo/fullquote"&gt;BLK.N&lt;/A&gt;, one of the  largest U.S. fixed income managers, on Thursday said it will  buy its far smaller competitor State Street Research    Management Co., marking the biggest takeover in the asset  management business this year.