Full access to private keys
This is only true for their "on chain" wallet and not their LN wallet, both of which are available within the app. The Blue Wallet LN wallet is custodial, meaning the operator of Blue Wallet has control and custody of the coin within an open LN channel. Blue Wallet advertises their LN wallet to be intended for only small amounts of coin.
While this is indeed true, BlueWallet Lightning wallet is custodial by default, you can connect it to your own node using LndHub:
https://github.com/BlueWallet/LndHubThis would make it completely non-custodial and an overall really great wallet for both on-chain and Lightning daily use.