> Has anyone found a non-criminal use case for blockchain yet?

As others has pointed out, DeFi is impressive, though still mostly fueled by speculation at this point. Digital collectables is another (think digital trading cards with limited supply). Might sound silly at first, but could prove an economic lever for artists and game designers.

> Preferably one that isn't better addressed by a single Postgres instance?

And here's the problem. Blockchain is expensive. A central, trustworthy server is going to be ridiculously inexpensive in comparison. For all the cost, the major benefit is resilience.