Could well be.

Although the 'Fort Blackmoor Prison Key' being in the game suggests to me that at some point Bethesda DID intend there to be a Fort Blackmoor.

The problem is back in the nineties, when creativity ruled over profits in PC gaming, they created a lot of lore. Daggerfall was waaay too big a game for the tech of the time. They anticipated a market that would keep on growing and a franchise that would expand in a certain direction. They had no idea games would be limited by marketing constraints and cross-platform porting. They had no idea that future playstations would be so weak compared to PCs, but would still dominate the market. To create a managable, rich 3d gameworld a lot of stuff has had to be dropped.

You make a good point. Certainly in the case of the size of cities which the in game books decribed, before they appeared in a TES game.

However, in the case of these smaller settlements or landmarks I see no reason why you wouldn't include them. Especially if they WERE previously mentioned in a book from a previous game. If I were designing the landscape of Skyim the first thing I'd have done is to go over, in detail, what had previously been alluded about Skyrim in in-game books and Lore, and build from there.

Yeah some good DLC should hopefully add some towns and cities removed from older lore!

Indeed. Or a modding opportunity for somebody.

Not that, as a 360 player, I'd benefit from that.

A DLC that would add towns, would just be weird. I mean those towns should have been in there already. No explanation would just make a town appear out of thin air.

Many Skyrim locations mentioned in the in-game books are nowhere to be found. Although most of these describe events that happened hundreds and hundreds of years ago. They could have easily be sacked or burned to the ground at some later time just like many places in medieval Europe simply vanished.

The in-game book series Beggar-Thief-Warrior-King is full of Skyrim locations including an entire jarldom that don't exist in the game. And yet they still included the jazabay graqes in Skyrim from the book.

Does anybody know which Jarldom that was?

The jarldom/kingdom is named Erolgard in the book "Beggar." There's a couple of copies of it in Riften if you want to pick it up in-game, one lying around in Haelga's Bunkhouse. Or you can just read it here http://www.uesp.net/wiki/Skyrim:Beggar

The other books which continue the tale "Thief" "Warrior" and "King" mention some other Skyrim locations. There is a Skyrim town named Jallenheim in the second book.
But the volcanic region described in the third book does actually appear in Skyrim but is named Eastmarch instead of Aalto on the maps.

Another long lost Skyrim location:

Falconstar. There is battle of Falconstar referred to in the in-game volumes Biography of The Wolf Queen and The Wolf Queen v7 (Both of which have appeared in Morrowind, Oblivion and Skyrim), which is Potema's last significant battle in the War of the Red Diamond. Her army wins, but she is informed as they do that her son the Emperor had been killed.

It refers to the enemy being holed up in the ruins of a Kogmenthist Castle. No specific details are given over the location of either.

The descriptions of Solitude throughout the Wolf Queen series, again, present Solitude as having a much greater scale, complete with a Mages Guild, a large market district and 'Castle Solitude'. Which is hard to reconcile entirely with the image of 'The Blue Palace' from Skyrim.

There is a fairly big castle in various states of disrepair that could be Darkmoor and changed names roughly to the west of Whiterun. It has been what 700 years or so since Tiber Septim was around? Could be it.

Yup. And given that even in different editions of the books which referred to it it changes from 'Blackmoor' to 'Darkmoor' Fort Greymoor is a likely candidate. Being just beyond Whiterun's Western Watchtower it's the likeliest candidate.

Odd that the game does HAVE a key for 'Darkmoor Prison' though.

Of course the fact that the hero of these books is named "Eslaf Erol", which is "false lore" backwards, might imply that even with Tamriel's extremely fluid definition of "research" these book should not be counted as anything but fiction....