The point is that transparency enables markets to accurately price risk and prevents the sort of market paralysis that caused the liquidity crunch.
Yes, it's true that people COULD have spent weeks figuring out the status of at least some of the CDOs on the market. But in reality no-one was reading 200 page long prospects that listed the exact mortgage originators and housing complexes of their CDOs. Buyers and sellers did not necessarily even have this data in peer-to-peer marketplaces. Financial institutions and their brokers bought securities based on their ratings, and dumped them ASAP once it became clear the entire sector was poisoned and they were doubtless overvalued.
> unless you force every contract to be public, blockchain technology will not help you that much
Smart contracts are public BECAUSE blockchains are open access. All parties can see the transfer of tokens on the network for the contracts that matter to them. No-one in the public necessarily knows what any contract represents, but if I am sold one I can monitor it in real-time. That was not possible with mortgage backed securities in 2007.