To solve this I ended the cdata block with a .Value and appended the code variable, then started a new cdata block with the rest of the multiline statement
ClientScript.RegisterClientScriptBlock(GetType(Page), "test", <![CDATA[
<script type="text/javascript">
testValue = ']]>.Value + Page.Title + <![CDATA[';
//more code
</script>
]]>.Value, True)