Sounds like you've already got it right there: implemented well a decentralized blockchain is basically a 3rd party as a transactional service (which through programmability can act as a ledger or escrow).

The commenter up above is directly referencing where existing 3rd party / escrow services are pretty terrible today and they're certainly not easily accessible to the general public or your average piece of computer code.

A lot of the pre-boom blockchain contract excitement really was as simple as "turn the awful banking middlemen into code so we can get the advantages of them programmatically."