I was not able to reproduce the problem using the 4.54 version of the MOSS RadEditor. I tried both Safari 2.x and 3.x and in both cases I was able to select the "New Window" value from the target dropdown, which automatically inserted _blank in the textbox above it.
If possible, send us some more information about your scenario - which Safari version do you use, exactly what steps do you make after opening the editor (e.g. select text, open link manager, etc.).
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