Adding the region role, as @rachel-fenichel proposed, seems sensible.
We've made it possible to tab to the Blockly workspace (and are working on refining this).
Developers can also add keyboard shortcuts of their own to go directly to the workspace (but we should document clearly how to set focus on the workspace correctly, so that their keyboard shortcut will behave the same as pressing tab the correct number of times.)
Expected Behavior
When a user loads the page they should be able to get to the workspace using only the keyboard.
Actual Behavior
The user has to click on the workspace, at which point they can't start using keyboard navigation.
Proposed solution