The following article in the official documentation explains how to customize the API to change the data returned to clients: https://world.episerver.com/documentation/developer-guides/content-delivery-api/how-to-customize-data-returned-to-clients/
We have followed the instructions in the article, and are able to change the returned data for pages and blocks when they are the content being requested through the API, e.g. "/api/episerver/v2.0/content/<page or block identifier>". The same goes for using friendly URLs for pages, e.g. "/path/to/page".
However, we have not found a way to change the data returned for blocks that are contained in a content area (either in a page or in a block). Does anyone have any tips for how we can go about doing this?