- You want right click on your blackboard in the Content Browser. Click “Asset Actions > Edit Selection In Property Matrix”.
- In the selection property matrix navigate to the “Details” panel.
- Then under “Blackboard”, you will find an array of keys. These keys can then be reordered like any array by clicking and dragging on the far-left-hand side of the array element.
- Once you’ve reordered your elements, click save.
- Make sure to close and reopen your blackboard (or behavior trees) through the Content Browser (or equivalent) and the changes you made should then be reflected!
Included an image for added context since I got a little lost figuring this out myself. Hopefully this helps someone else 
. It’s a little odd that you can’t update the keys order directly within the blackboard, but at least there is a way even if it is a little hidden. Categorizing and ordering variables in blackboards would both be welcome QoL features.