Blue Chips Are Flat, Caterpillar Gains  NEW YORK (Reuters) - U.S. blue chips were little changed on  Tuesday as an upgrade on Caterpillar Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=CAT.N target=/stocks/quickinfo/fullquote"&gt;CAT.N&lt;/A&gt; and a  three-session drop in oil prices offset weakness in technology  shares.