I see what you're saying, there is no option to go from steem --> BTC on Blocktrades or Shapeshift for that matter. It appears for now, that the only way to convert your steem to bitcoin is at an exchange, like open ledger or bittrex. You have to deposit your steem into one of those exchanges and then trade it into bitcoin.
I can't speak for openledger since I have never used it, but at Bittrex, when you click the '+' sign indicating you want to deposit steem into your steem wallet, it will prompt you for your steemit account name and public memo key, which can found under your permissions on steemit.com. Once you've sent it and your bittrex wallet shows a steem balance, you can sell it for BTC.
Sorry, it looks like that's the only way to do it, right now.