Firstly those address were generated because BlueWallet is an HD wallet and HD wallets always have 20 extra free addresses to use for a new transactions.
Here comes the question:
1. If I happen to use any of the generated addresses from BlueWallet that are attached to my phrase, can I spend them using my trust wallet? Since I can still spend anything from my old address using a blue wallet or even my electrum, trust wallet allows me to do the same thing, as I can't fine any of such options on trust wallet.
Yes they will still be visible to your wallet on trust wallet because the bitcoins are actually associated to that key or seed phrase. So any thing going out from that key will be visible in trustwallet.
2. Among the new addresses that were generated, if I happen to receive new coins into them and I want to spend just the coin I received with the new address, leaving the old one that came with it when I imported the address, will it be possible for me to specify which address I want to be debited from? And if yes, will my transactions also be linked to my old bitcoin address, which I have been using in the past?
These are just questions running through my head out of curiosity.
Yes you do this by using coin control to freeze the other address but make sure the amount to spend is will be covered by that address including the transaction fees.
The transaction will not be related to old wallet because only the new wallet will be visible on the public blockchain and that’s why it is advised to use different addresses to receive bitcoin