Hi MSZ,
if you need the blocking response from the ASM in JSON format, just modify the default response message. You can just send JSON instead on HTML content.
I.e.
[{"status":"error","data":{"message":"This request was rejected due to a violation. Please contact webmaster@example.com, and provide this reference ID: <%TS.request.ID()%>"}}]
Please make sure, that you change the Content-Type Header as well.
If you cannot set the JSON response for the whole Security Policy, but for single URIs or if the request is of type "application/json", you need to create an iRule, which sends the response instead of the policy itself.
Hope this helps.
Greets,
svs