custom response per uri with jason code

You can do this in an iRule triggered on

when ASM_REQUEST_VIOLATION {
   ...
   if {[HTTP::uri starts_with "/blabla"} {
      HTTP::respond 500 content {{"error":"invalid character", errorCode: "1231231"} } Header {Content-Type} {application/json}
   }
   ...
}

Otherwise, you need to create a new ASM policy that sets the Violation response to an HTTP 500 response as required. Use a Local Traffic Policy to select between the two ASM policies based on the HTTP::uri.