I checked this out extensively; it (claims) not to collect data unless you enable one of the settings which blocks adult sites.
From the extension options:
> Enable/disable the option to block sites in bulk by category. This allows BlockSite to access information about the sites you visit, for use as explained in our Privacy Policy.
And from the privacy policy itself:
> You may opt out of the automatic collection and sharing of Non-Personal Information described above as follows:
> For the BlockSite browser add-on extension:
> By deselecting the checkbox next to the words “Block Adult Sites” which can be found under the Settings tab of the Options page for the BlockSite extension.
From the extension options:
> Enable/disable the option to block sites in bulk by category. This allows BlockSite to access information about the sites you visit, for use as explained in our Privacy Policy.
And from the privacy policy itself:
> You may opt out of the automatic collection and sharing of Non-Personal Information described above as follows:
> For the BlockSite browser add-on extension:
> By deselecting the checkbox next to the words “Block Adult Sites” which can be found under the Settings tab of the Options page for the BlockSite extension.