Does Bisq somehow use the Bitcoin blockchain for recording trades?
When fulfilling a trade the BTC seller deposit the coins in a multisig script with 2 keys, one controlled by them and one by the buyer.
When the BTC seller acknowledge the reception of the fiat payment, they release the coin to a script only controlled by the buyer.
This mechanism, as well as a number of others (see the documentation for details), aims to disincentivize fraud.