So what of a code would be part of Bitcoin protocol and that would depend on that the network is running that could sign blindly the blocks.. (Putting like a seal on them), include in the blocks a cryptographic signature that works like authentication signature ?
Your idea of a built-in signature is basically what blockchain cryptography already tries to do. It doesn't magically solve the problem of a complete network shutdown and potential malicious actors afterward. You'd still need a way to pick the "good" chain after the chaos.