Misbehaviour for some scenarios Rated 4 out of 5 stars
It seems the addon get confused for some scenarios:
1) I have blocked "mail.google.com" by accessing Gmail and using the "Block This!" context menu option. That alone works great.
2) Apparently, other *.google.com domais also got blocked. Tested with translate.google.com and docs.google.com. They don't show on Recently Closed Tabs nor History.
Also, I have tested this scneario with others sub-domains (a.xpto.com, b.xpto.com) with same results.
I don't know why, but it seems the extension somehow add two hash entries when you add only one URL. Ex. when I block mail.google.com (and only that one) I could see two entries on "extensions.historyblock.stringpref":
baea954b95731c68ae6e45bd1e252eb4560cdc45 - hash for google.com
da39a3ee5e6b4b0d3255bfef95601890afd80709 - hash for "empty string"
So, I cleaned that registry, went to www.fileformat.info/tool/hash.htm, calculated the SHA-1 hash from mail.google.com and entered that value (1fe099df5f46e4c3c7e8ff6d5a72f695a06acad4) on the registry.
Apparently is working great now.