Air-gapped crypto wallet.
I wouldn't say it's a black box system. If you want, you can read the code here, compile it yourself and load it on to your phone - https://github.com/paritytech/parity-signer
NOTE: The number of mentions on this list indicates mentions on common posts plus user suggested alternatives. Hence, a higher number means a more popular project.
Any plans for Hardware Wallet Intergration?
1 project | reddit.com/r/AcalaNetwork | 19 Apr 2022
Signing Parachain Transactions with Parity Signer
2 projects | reddit.com/r/Polkadot | 13 Feb 2022
polkadot js crowdloan security
1 project | reddit.com/r/dot | 10 Jan 2022
1 project | reddit.com/r/Polkadot | 1 Mar 2021
WARNING: Risk of losing funds, please do not use this Parity Signer · Issue #724
2 projects | reddit.com/r/polkadot_market | 13 Feb 2021