Maybe the option wasn't there back when you tested it out. Maybe they added it later or you forgot to activate the advanced options.
I just tested it.
Bluewallet allows you to add a passphrase to the seed phrase only when you import a seed phrase. It doesn't allow you to add a passphrase to the seed phrase when you generate a seed phrase.
Have you tried generating a new Blue Wallet native seed, delete the wallet, and then recover that same wallet from seed with advanced options enabled to get to the part where they allow you to add a passphrase?
This is the only way to have a wallet with a seed phrase generated by bluewallet it self and extended by a passphrase.
You should generate a seed phrase in bluewallet and then create a new wallet with importing that seed phrase and entering your passphrase.