add class to parent

<!DOCTYPE html>
<html>
<head>
<style>


body{
    font-family: Arial, Helvetica, sans-serif;
}

.tableClean {
  border-collapse: collapse;
  color:#555555;
}

.tableClean tr{
    border-left: 1px solid #cccccc;
    border-right: 1px solid #cccccc;
}

.tableClean tr:last-child{
    border-bottom: 1px solid #cccccc;
}

.tableClean tr:first-child{
    border-top: 1px solid #cccccc;
    background-color: #cccccc;
}

.tableClean td, .tableClean th{
    padding:0.75em;
}

.tdBlue{
    color:#000055;
    background-color: cornflowerblue;
}

.trGreen{
    color:#005500;
    background-color: darkseagreen;
}

</style>
</head>
<body>

<table class="tableClean">
  <tr>
    <th>Header A</th>
    <th>Header B</th>
    <th>Header C</th>
  </tr>
  <tr>
    <td>Content 1a</td>
    <td>Content 1b</td>
    <td class="tdBlue">.tdBlue</td>
  </tr>
  <tr>
    <td>Content 2a</td>
    <td>Content 2b</td>
    <td>Content 2c</td>
  </tr>
</table>

<script>

var thisSourceList = document.querySelectorAll(".tdBlue");
for(var i = 0; i < thisSourceList.length; i++) {
    thisSourceList[i].parentNode.classList.add("trGreen");
}

</script>

</body>
</html>