Passphrases can be added in Blue Wallet if you activate advanced mode in the settings. It's explained somewhat
here.
Blue Wallet also has a
Plausible Deniability feature where you can set up a fake wallet. In case of a $5 wrench attack, you can unlock that fake wallet with your secondary password and not reveal the main one.
When I used it to create wallet directly on Bluewallet, I did not see anything like I should include passphrase, but I can be able to select the wallet type, or to provide entropy (which would be similar to throwing of dice).
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. Or am I getting something wrong?