I have five solutions, most of which involve code. The first solution is probably the easiest for you:
First solution
These MSDN articles tell you how to modify ItemStyle.xsl and add your own CQWP style to it (http://www.microsoft.com/belux/msdn/nl/community/columns/stevenvandecraen/contentquerywebpart.mspx, http://msdn.microsoft.com/en-us/library/bb447557.aspx) You should be able to override the code which displays hyperlinks and insert target=”_blank”.
Second solution
This link to Heather Solomon is helpful also http://www.heathersolomon.com/blog/articles/CustomItemStyle.aspx
Third solution
The next suggestion is to use javascript to find all links in the Content Query Web Part and set their targets after the page has loaded. Probably an icky solution for you but possible.
Forth solution
You might use Visual Studio to create a webpart which inherits from the Content Query Web Part and writes all hyperlinks with new browser targets. See google.
Fifth solution
My last suggestion is to get something like the Enhanced Content Query Web Part and see if that will do it for you (http://www.codeplex.com/ECQWP)
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