I can bring some light into this
First things first, the need of “locking” the app thumbnail comes from a security point of view. In order to offer a smooth app switcher iOS stores a screenshot of how every app looked before they were closed. Since those screenshots are not in control of our app (they happen at OS level) we don’t want to potentially compromise your data, so we hide it right before iOS takes the snap.
There are many ways of doing this. One would be the “1Password approach”, show a lock screen or splash screen just presenting the app icon or similar. This approach works best when the app has some sort of login, so every time you open it you accept that there’s a barrier.
The second approach (the current blur) comes from the idea that the app should always be accessible (like your email or Messages are) so when you switch apps you want at least to have a hint of what were you doing before switching apps (maybe you were checking a transaction, or a report, or talking with customer support) so when the app unblurs there’s not so much friction or jump from the “locked” screen to the actual app.
I appreciate is not ideal but it’s the best solution we imagined. Actually, after building it we realised that it’s exactly the same pattern Apple uses on their own iTunes Connect app… so I guess there’s not really too much room for improvement until iOS provides new/secure ways of dealing with this.