Blackholing your canister simply means removing all controllers, and ensuring the canister has no way of being upgraded. Since blackholing a canister is more for others than the creator (trust), usually open sourcing the code of the canister smart contract and providing some sort of reproducible build is recommended.
https://cycleops.dev blackholed its core canister monitoring smart contract so that others can easily integrate with it in a trustless manner. Here’s a link to the monitoring canister repo, which includes different versions of our blackhole, as well as a way to verify the canister wasm and having 0 controllers via a GitHub action.
Note: We needed to deploy different versions of our blackhole to release new functionality, since a blackhole canister cannot be updated once blackholed.