codice:
function viewField(id_field){
  document.getElementById(id_field).style.visibility = "visible";
}

<input name="blob" type="file" id="blob" class="testo" size="55" 
onclick="viewField('blob2')">

<input name="blob2" type="file" id="blob2" class="testo" size="55" style="visibility: hidden" 
onclick="viewField('blob3')">

<input name="blob3" type="file" id="blob3" class="testo" size="55" style="visibility: hidden" 
onclick="viewField('blob4')">

<input name="blob4" type="file" id="blob4" class="testo" size="55" style="visibility: hidden">