Azure ad b2c custom connector "ShowBlockPage" response is not working - azure-ad-b2c

Question

I am following the documentation here to return the blocking response https://learn.microsoft.com/en-us/azure/active-directory-b2c/add-api-connector?pivots=b2c-user-flow#example-of-a-blocking-response from api connector to azure ad b2c, however even after constructing the right response as shown in the documentation, I am still not able to show the blocking page for b2c user flow.
Note that this connector gets invoked at sign-in.
I have verified that the response from api which seems correct and looks like below
{
"version": "1.0.0",
"action": "ShowBlockPage",
"userMessage": "You must have a local account registered for Contoso."
}
With this, was hoping to see a blocking page as below (screenshot from docs) but b2c does not show it and goes straight to the connected application.
What did I miss? any pointers would be appreciated. TIA.
Here is my api connector's code
public static async Task<IActionResult> Run(HttpRequest req, ILogger log)
{
log.LogInformation("C# HTTP trigger function processed a request.");
// Get the request body
string requestBody = await new StreamReader(req.Body).ReadToEndAsync();
dynamic data = JsonConvert.DeserializeObject(requestBody);
// If input data is null, show block page
if (data == null)
{
return (ActionResult)new OkObjectResult(new ResponseContent("ShowBlockPage", "There was a problem with your request."));
}
// Print out the request body
log.LogInformation("Request body: " + requestBody);
// check for issuer
if(data.identities != null)
{
string issuer = data.identities[0].issuer;
log.LogInformation("issuer detected: " + issuer);
if(issuer == "github.com")
{
log.LogInformation("Returning an error!");
//return (ActionResult)new BadRequestObjectResult(new ResponseContent("ValidationError", "Please provide a Display Name with at least five characters."));
return (ActionResult)new OkObjectResult(new ResponseContent("ShowBlockPage", "You must have a local account registered for Contoso."));
}
}
// Validation passed successfully, return `Allow` response.
return (ActionResult)new OkObjectResult(new ResponseContent()
{
jobTitle = "This value return by the API Connector"//,
// You can also return custom claims using extension properties.
//extension_CustomClaim = "my custom claim response"
});
}
and here is the ResponseContent class
public class ResponseContent
{
public const string ApiVersion = "1.0.0";
public ResponseContent()
{
this.version = ResponseContent.ApiVersion;
this.action = "Continue";
}
public ResponseContent(string action, string userMessage)
{
this.version = ResponseContent.ApiVersion;
this.action = action;
this.userMessage = userMessage;
if (action == "ValidationError")
{
this.status = "400";
}
}
public string version { get; }
public string action { get; set; }
[JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
public string userMessage { get; set; }
[JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
public string status { get; set; }
[JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
public string jobTitle { get; set; }
}

Answer 1

We ended up creating Microsoft support ticket for this and got response that this is by design. ShowBlockPage response works only with sign-up user flows and not with sign-in user flows.