Blue Chips Extend Losses, Wal-Mart Weighs  NEW YORK (Reuters) - Blue-chip stocks extended losses on  Monday as a disappointing start to holiday sales from the  world's largest retailer 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; weighed  on sentiment.