Hi Jack,
No need to be sorry for not understanding. Roles and permissions are definitely not easy to understand!
You only need to create one role (as described in my previous post) then assign it as follows:
Turn editing on in the course then click the edit cog for the HTML block and select 'Assign roles in HTML block'
Assign the block viewer role to Jack
In the administration block, follow the permissions link
Remove Authenticated user, Guest and Student as roles with permission to view the block
Then only Jack will be able to view the HTML block.
Do the same for the Online users block. The block viewer role will be available for you to assign to John. Because you are assigning the role in the block context, it only applies to the block.
Hopefully you can follow these steps. Please shout if anything is unclear.