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This extension will not upload anything to a server. Not even configuration data. The people you blacklist have no means of knowing you've blacklisted them, and neither do anyone else, unless they are using your computer.
In the future, the extension may be extened by a crowd-source feature, allowing you to share blacklists in a cloud. But if this happens, the extension will ask you first. All future multi-user features will be designed in an opt-in fashion.
See the permissions page for a more technical discussion of the privacy aspects of this extension.