Can someone point me at the implementation of custom fields and how they are stored?

The core custom field implementation is in phabricator/src/infrastructure/customfield/. Storage depends on the field subclasses.

This task is very complex, and it's unlikely a contributor can make much headway without existing experience in the codebase.

Given the level of interest in the community, I suggest starting from the field types that are easier to extend. I know it might not be a superb way, but most it would perhaps provide a way forward.

Side: how can I have blocking tasks in my installation of Phabricator?

Given the level of interest in the community

There isn't much interest in this feature.

I suggest starting from the field types that are easier to extend.

The field types are hard to extend because they support lots and lots of features like this one. The only way to make them easier to deal with would be to remove features.

Side: how can I have blocking tasks in my installation of Phabricator?

There is no way to disable them, so you should have them. You can use this link to edit blocking tasks: