You could simply pass through the value to the function doUpdateEntries
as another parameter.
<script type="text/javascript">
function doUpdateEntries(id, value) {
alert(id);
alert(value);
}
</script>
while ($row = mysql_fetch_assoc($result)) {
<input onblur="doUpdateEntries(this.id, this.value);" id=".$row['entry_id']." name=".$row['entry_id']." type="text" value=".$row['reviews'].">
}