The name "_new" is NOT illegal (as stated above); it is simply not a reserved name. If you open a new window, you can name it so that if another link tries to open a window of the same name the contents of that window will be replaced. This has its uses. One can use target="foobar" and if no window named "foobar" exists, one will be created.
But it is correct that "_blank" should be used to ensure that a new window is opened every time the targeted link is clicked.