The tutorials are nice but have nothing about blobs in them and handling of them seems to be separate from the text portions of the document.
The tutorials do cover how you create and update the images via Couchbase Lite using the setBlob API and how it get’s synced over to the sync gateway side. It also describes how can update the documents via sync gateway REST API.
var url = $“{baseUrl}/blob_%2f{imageName}?_rev={revision}”;
You seem to have an underscore before the rev… I would suggest printing out the url and testing it directly with a http client like curl/postman to ensure it works.