A Signer is a component that performs cryptographic operations, separately from a wallet. A Bitcoin hardware wallet is an example of a Signer, where private keys are controlled on a hardened device. There is currently no complete solution for a hardware signer for the Lightning network.
A Blind Signer is a signer that does not perform validation. There are several consumer Lightning wallets and node implementations that as of today support only blind signing. We believe these configurations are insecure.
A Validating Signer performs a comprehensive set of checks of validation to ensure that the keys are not misused. For example, a validating Bitcoin hardware wallet checks the destination, amount and change outputs in collaboration with the user.
A layer-2 validating signer is significantly more complex, because of the complexity of the Lightning protocol.
While a Blind Signer is a technical step on the road to the higher security of a Validating Signer, by itself it actually reduces security if deployed in production. This is because it presents two points of attack - at the node and at the signer.