Quick evaluation:
Instead of inserting an iFrame via Blocs HTML widget, just instead insert a plain DIV for the Shopify JS code you placed in the page head too reference. As discussed here: “Add script tags separately”.
So in your example case, it would just be the following that would need to be pasted into the HTML widget inside of Blocs.
<div id="product-component-15e96f08ee2"</div>
The “https://sdks.shopifycdn.com/buy-button/latest/buy-button-storefront.min.js” seems to work by using that DIV reference and creating its own iFrame. So when your using an iFrame in Blocs like you tried - vs - just the straight DIV approach described above, that is where the stacked or double iFrames occur that you’re experiencing and see.
Hope it helps your efforts and testing.
Blocs_User