I enabled the blackbox extension (http://mercurial.selenic.com/wiki/BlackboxExtension) using the TortoiseHg File, Settings, global settings dialog.
Now when I interact with the repository using TortoiseHg the blackbox.log file within the .hg directory of that repository is not created / written to. When working on the same repository using the command line and the hg.exe (from the TortoiseHg installation directory) the blackbox.log file is created and the commands are logged in that file.
I tested this on Windows 7 (32 and 64 bit, see below). For the 32bit tests I used a clean virtual machine where no other SVC / hg etc. tools were installed. TortoiseHg 2.9 (64 bit) with Mercurial 2.7 TortoiseHg 2.9 (32 bit) with Mercurial 2.7 TortoiseHg 2.8.2 (32 bit) with Mercurial 2.6.3 Nightly Build tortoisehg-stable-126.96.36.199-x86-bf565e1c3178.msi
When enabling a build in extension (e.g. blackbox) this extension should work when using the command line AND when using TortoiseHg.