I think Blockfolio is a great app. Yes, I've had concerns regarding data privacy, as we all should be these days.
Thank you for the detailed clarification. 
I'd like to know know how Blockfolio is funded?
As with many apps, they have to get funding somewhere. Many are ad funded, but I don't recall seeing ads on BF.
I'd gladly pay for this app service if I knew my data was secure.