It appears that you cannot add a block to the "Add Block" list once you are editing an email. To get around this go to an email that has the desired block and copy the HTML of that block, then insert any block into your email. Edit the new block and paste the copied HTML over the default HTML. Then you can copy and reuse that block, only it will show up with the wrong block title during editing which has no effect on the actual email.
Hopefully this weakness in CC will be fixed soon.
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