Hola:
A ver si me sale:
<html>
<head>
<script>
var visible = true;
function parpadear() {
document.forms["formulario"]["parpadeo"].style.backgroundColor = (visible) ? "black": "white";
visible = !visible;
}
</script>
</head>
<body onload="setInterval('parpadear()', 200)" >
...
<form name="formulario" >
...
<input type="text" name="parpadeo" value="prueba" />
</form>
...
</body>
</html>
Saludos
