Hi ​@LukeR,

Here is an example of how to achieve your solution using the JS-Insert plugin from the gallery.

I will start by adding a Text-Short control to my form.
I have given it a default value (optional).
Add a CSS class of base64Input.  This is a must so that jQuery can reference the control.
 

Next, I have added a label control and given it a CSS class of decodedOutput.
 

Add the JS-Insert control.
Configure it to be visible No.
JS to execute is:

  $('.base64Input input').on('blur', function() {
    var base64String = $(this).val().trim();
    try {
      var decodedString = atob(base64String);
      $('.decodedOutput').text(decodedString);
    } catch (e) {
      $('.decodedOutput').text('Invalid Base64 string.');
    }
  });

Test:
On blur, the form will look like this:
 

This is what the form looks like in the designer.
 

Thanks, Simon! That’s exactly what I was needing!