show method problem ?

//does'nt work (show undefined)

<div id="bla" style="display:none;">ok</div>
<script language="javascript">
$("bla").show();
</script>


//this WORK

<div id="bla" style="display:none;">ok</div>
<script language="javascript">
$("bla").style.display = '';
</script>

can help me ?