The problem is not that the fax itself is black but that the Windows XP viewer cannot handle certain types of TIF file and rather than give a message, it just displays black.
There are other viewers that work if you get this problem - Acrobat (full version) seems to handle them well and the Windows 2000 picture viewer is also ok.
This is a free viewer that also works well:
Voipfone Customer Services
iNet Telecoms Ltd (Voipfone)
227 Marsh Wall
Registered number: 05168033
Vat Registration Number 858850966
Telephone: 020 7043 5555
Fax: 020 7043 5556