When I used it to create wallet directly on Bluewallet, I did not see anything like I should include passphrase.
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 import a seed phrase from Electrum to Bluewallet with the advance option checked, it did allow the import of seed phrase with passphrase requested for too. But no option to do that when using Bluewallet to generate keys and addresses directly.
That's kind of weird. Why would they allow you to extend an imported seed with a passphrase but not allow the same thing with one natively created through their own wallet? 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? I guess when you do that, you can keep using the new addresses from the passphrase-protected wallet from that point on. It would be worth trying it out.