I believe that the writers went about as far in that direction as they could. In
Fable: The Lost Chapters, we meet Scythe. He is wearing golden mail and a blue cloak. As Roger said, the Fable Wiki refers to William Black having disappeared "wrapped...in golden mail and a royal blue cloak"; the information on the Wiki is taken from another source (unfortunately the Wiki doesn't have the source listed), which I do not recall off the top of my head, but I distinctly remember that it was an official backstory for William Black.
In
Fable II, if you choose the 'Family' option, Rose writes you a letter. In the letter, she claims to be with a man who seems to be a king, but who wears a dark hood. This has been generally accepted as referring to Scythe.
In
Fable IV, Theresa tells the story of William Black to the protagonist; how he tried to recreate the world, and it nearly destroyed him. This would explain Scythe's withered appearance (in addition to the fact that he has been alive for quite some time). In addition to this information, we have the remark made by the Oracle in
Fable: The Lost Chapters, telling us that the only person who has "seen and done more" than Scythe is Jack of Blades. This would make sense if Scythe and William Black were the same person, because if Jack of Blades is the only Hero older than Scythe in
Fable: The Lost Chapters, it would imply that Scythe was around at least near the same time as William Black.
Given all the above information, I think it is a safe bet that William Black is, or rather, became Scythe. You may find the concept art for Scythe interesting (it contains no information directly linking him to William Black, other than his attire, but it sometimes helps to have a visual):