US Blue Chips Fall After Merck Recall  NEW YORK (Reuters) - The blue-chip Dow Jones average was  knocked lower on Thursday after drug maker Merck   Co.&lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=MRK.N target=/stocks/quickinfo/fullquote"&gt;MRK.N&lt;/A&gt;,  a member of the Dow, recalled its widely sold Vioxx painkiller  and oil prices hovered around \$50 a barrel.