Blue Chips Drop After Merck Announcement  NEW YORK (Reuters) - U.S. blue chips were lower on Thursday  after drug company and Dow component 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; said it  was withdrawing a key drug, casting a dark mood over the market  as oil prices remained above \$49 a barrel.