This is on Windows 7 using the recent Snapshot ef4a40 (as well as several before; this issue exists for some time …)
Adding a .exe to the blacklist via the Add button a file-selection window will open, where you can only select existing (.exe) files.
The exe will then be added with it's full path.
The overlay will still display; probably because it uses the full path, in contrary to the default entries, which only use executable names.
This issue was with CoD4, iw3mp.exe, for me.
As punkbuster seems to kick for the mumble overlay, adding this as a default entry is debatable; as it's only an issue with PB enabled; which it is for most ppl. probably though …