I think the BlockRenderer should be refactored to BlockManager
Ger-Jan can you please add documentation (as comment or attached file) how to use this feature in a block?
In the content (detail) controller the line $this->blockManager->setDocument($article); registers the active document.
To get the active document in the block you can pass the BlockManager to the BlockHandler constructor. In the execute function you can call $this->blockManager->getDocument(); to get the document.