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);
})();
It reduces complexity and a global, which is far more important.
And that blink loop is probably not that big - just how many blink tags do you have?
I don't use it for blink loops anyway, I use it for toggle on/off, where keeping state for each element individually is pretty important.