You're accessing the DOM every iteration, so I'd bet that it does matter somewhat if blink is used a lot.

You can avoid polluting by make it an actual function and then setting the function.blinkOn instead.

You're right: individually swapping them this works better, but the intent is to have them all off or all on.

  (function() {
    var blinkIterator = function() {
        var blinks = document.getElementsByTagName("blink"),
         onOff = blinkIterator.blinkOn ? "" : "hidden";
        for (var i = 0, l = blinks.length; i < l;)
            blinks[i++].style.visibility = onOff;
        blinkIterator.blinkOn ^= 1;
    };
    blinkIterator.blinkOn = 1;
    window.setInterval(blinkIterator, 750);
  })();