If you want 100% safety then I will suggest bitcoin core or paper wallet where only you have control of your keys, with other wallets your keys are accessible by wallet developers
Blockchain is actually believed widely among bitcoin users but we never know their wallets may get hacked sometimes i dont know, so it is always better not to keep a lot of coins in an online wallet like that and just keep them in a hardware wallet.
Their services got hacked few times long time ago, there's critical bug when create new address in mobile application, their services often down these days and i heard some people have problem if they activate 2FA.
Those what i mentioned already fixed, but who knows if it's still safe to keep your bitcoin there.
it is obviously never safe keeping your coins in an online wallet if you know they are worth a lot because there is no guarantee about any online wallet if when they may get hacked or when your coins are stolen, so better to keep your coins in a cold wallet if they are a lot.