I have five solutions, most of which involve code. The first solution is probably the easiest for you:
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”.
This link to Heather Solomon is helpful also http://www.heathersolomon.com/blog/articles/CustomItemStyle.aspx
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.
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)