C++ download audio/video file from blob URL?

I have an Angular front end with a C++ powered back end. I want the C++ back end to grab a file from a blob URL created by the front end (with URL.CreateObjectURL).

I have tried using URLDownloadToFile:

HRESULT hr = URLDownloadToFile(NULL, theBlobURL, outfilename, 0, NULL);

As well as curl:

                CURL* curl;
                FILE* fp;
                CURLcode res;

                curl = curl_easy_init();
                if (curl) {
                    fp = fopen(outfilename2, "wb");
                    curl_easy_setopt(curl, CURLOPT_URL, theBlobURL);
                    curl_easy_setopt(curl, CURLOPT_WRITEFUNCTION, NULL);
                    curl_easy_setopt(curl, CURLOPT_WRITEDATA, fp);

                    res = curl_easy_perform(curl);
                    if (res != CURLE_OK)
                        fprintf(stderr, "curl_easy_perform() failed: %s\n",
                            curl_easy_strerror(res));

                    curl_easy_cleanup(curl);
                }

Both of these methods work with a traditional file URL (I tested with this public file), but fail with my blob URL. URLDownloadToFile gives "the specified protocol is unknown" as the HRESULT and curl saus "CURLE_COULDNT_RESOLVE_HOST".

I've confirmed via the browser that the blob URL is still available at the time I am trying to open it.

Do I need to do anything different to get a blob?

Remy Lebeau was correct, the Blob URL is private to the browser. It cannot be downloaded outside the browser.