Blue Chips Slip as Wal-Mart Disappoints  NEW YORK (Reuters) - U.S. blue chips ended lower on Monday  as a disappointing sales forecast from Wal-Mart Stores Inc.  &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=WMT.N target=/stocks/quickinfo/fullquote"&gt;WMT.N&lt;/A&gt; cast a pall over sentiment, overshadowing a two-session  pullback in oil prices.