I don't think drawing just a block diagram of system means giving away the trade secrets of your product. I didn't ask him to draw flow charts of algorithms and business logic. And I don't even do that. I have seen many interviews purposefully conducted by rival companies where interviewers are specifically asked by company's marketing team to drill down interviewee on particular functionality and get details of business algorithms, system performance etc. Offcourse this happens when there is cut-throat competition and few players.