Blue Chips Open Down; Oil Near \$48  NEW YORK (Reuters) - Blue-chip stocks opened slightly lower  on Thursday, with rising energy prices feeding investor  concerns, but the Nasdaq index opened higher after software  company Cognos Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=COGN.O target=/stocks/quickinfo/fullquote"&gt;COGN.O&lt;/A&gt; posted better-than-expected  earnings.