Currently you have to convert the reusable block in to a regular block – Select your reusable block and from the 3 dot menu in its toolbar select Convert to Re-usable Block. Which is what you should do if you want to make a unique block.
If you want to style a re-useable block ( ie. use the same block in multiple places ) then go to the Top Bar 3 dot menu – select Manage All Reusable Blocks and edit the block there.