Secure Elements are closed-source chips that are integrated into hardware wallets to protect them from physical attacks. They are usually covered by NDAs, increase the cost of the hardware wallet, and prevent users from building their own device.
Instead of a security element, Blockstream Jade uses a unique security model that allows it to remain fully open source while being protected from physical attacks and achieving similar (if not better) security from this potential threat - by acting as a "virtual" Secure Element.
The blind oracle model that Jade uses is completely open source and truly blind. It knows nothing about the Jade Wallet data and does not even know the user's actual PIN. Users can use Blockstream's blind oracle to protect their wallet, or they can use their own.