Blue wallet is 100% safe because is opened source and no vulnerability?
It's as safe as your device. If your phone is infected with malware, you can't expect your wallet to remain safe but yes, the developers of the wallet cannot access everyone's funds.
I knew about android phones since version 1.0 but till this day I have never seen a n android phone getting infected with malware, I don't think it's possible but correct me if I am wrong.