There are no such option, its a little tricky since blackfolio is not a full accounting fortfolio.
You have do it manual if you want to shown your btc
When you sell your altcoin, your alt qty will reduce and you gain btc in return
as blockfolio didnt directly count the btc that you gain for that transaction, you have to input (buy) btc manually against fiat
I don't have BTC - I sold my altcoins for USDT.
I guess the bottom line is that Blockfolio can't handle selling alt coins for USDT or USD and has no way of dealing with my transactions.
In my case then, it seems Blockfolio is only useful to keep track of market value on cryptocurrency.