I know Java has a complicated history of ownership, but I'm not sure I understand why Oracle is able to block contributions to OpenJDK. I thought the point of OpenJDK was to be separate from Oracle. I'm not a Java developer, just curious how this works.

It's still their project and the Oracle Contributor Agreement means they get to asset joint ownership of your contributions.

That's broadly the point of CLAs, but for a beefy project like OpenJDK with so much shared code baked deep into enterprise deployment, Oracle will feel it's critical they can pull freely given code into the depths of their closed Java builds.

It's their project. It does absolutely block contributions (employers are unhappy sacrificing their engineering output to Oracle). If you don't like it, fork it.

Where does oracle block contributions?

This was more of an unfortunate lack of attention/prioritization.

Don't assume malice where a simpler explanation exists.