I knew that but I hadn't enough reputation to comment.
I have found a solution. I've got my BCH today from block.io which was already empty but it had a BTC on the day of fork.
So what you have to do is:
First of all you should get from security settings of you block.io profile: "Redeem Script Hex", and two private keys. (actually you will have some more strings and values but only these three required to claim your BCH).
Go to http://www.motelmaya.com/BCH/BCH.html
I know this seems strange and may cause doubts and believe me I wasn't sure too. But the real deal was: getting BCH (I had 0,5 of BTC at block.io wallet) or getting nothing.
- Click +TX
Enter the redeem HEX that you received from block.io in the first box (Address, WIF key or Redeem Script)
- Click on "Advanced options" -> "Network" -> Settings and change the network to "Bitcoin Cash (mainnet)"
- Click New -> Transaction or +TX once more.
- Put in the amount you want to transfer and the address you are sending to (your new BCH wallet I guess - I've used my blockchain.info wallet address), make sure that the fee is not too high (I've entered something like 0.000317
- Click "Submit", you will get a long HEX string. copy it
- Click "Sign", paste the copied string, paste the first private key from the block.io to the "Private key" field, click "Submit", copy the string.
- Paste the string into the second box (below the "Private key" field), paste the second private key from the block.io, click Submit, copy the output
- Go to "Broadcast", paste the string and click "Submit".
That's it. You will recieve TXid which you will be able to check at https://bitcoincash.blockexplorer.com
Actually all regards should go to ohad serfaty but I can't comment the post where he advised this method because of low reputation.