It doesn't ensure that the data is correct, valid, hasn't been tampered with, or even entered.

A supply chain involves 3rd parties who have nothing to gain from this. If the issue is the middle man between A and C, blockchain isn't solving any problem. The distrust is with B, having a blockchained backed paper trail doesn't provide anything a database gives you right now.

You're asking for a single source of truth between A B and C. And saying you cannot trust A B or C, so you want blockchain to solve it, which needs to be created by someone, so you introduce D and now you have a central governing body who dictates everything for A B and C. Or you are A and you say 'hey B and C, you need to use this system we created cos we don't trust you!'.

Blockchain solves nothing in the supply chain.

It ensures that data hasn't been tampered with after initial entry. And that a uniquely attributable digital signature gets recorded at the point of entry, along with a timestamp.