I had this exact same issue, except that instead of "/blog" my collection parent url is "/episodes". However, I've created a custom landing page for my podcast located at "/listen" and I don't want visitors to see "/episodes" since it's not designed.
The regular URL Mapping provided by Squarespace doesn't work, since it is a collection parent.
I was finally able to work around this by injecting the following page header code into the Page Header.
<script>
if(window.location.href == 'https://compelledpodcast.com/episodes' || window.location.href == 'https://compelledpodcast.com/episodes/') {
window.location.href = "https://compelledpodcast.com/listen";
}
</script>
I hope that helps someone else!