Here is a quote from page 24 of the DAB Campaign Sourcebook (later repeated in the 4e book):
"High Thonian Lands: When the
Valley of the Kings stood strong, the
High Thonian nobles ruled much of
present-day Blackmoor. The bloodline’s
fall, as well as invasion and civil war,
eroded much of what was once the unified kingdom of
Blackmoor."

Valley of the Kings? Does this ring any bells for anyone from some other references?

This sounds like something of an alternate history, maybe not unlike the history seen in the D20 Dungeons of Castle Blackmoor. If feels like it comes from something Arneson might have said since I don't see how it could tie to TSR's DA material.

The text says "unified kingdom of Blackmoor", so the Valley of the kings would seem to be in Blackmoor (not Thonia or some other place) and it must be some sort of powerbase with fortreses or cities.

Perhaps Vestfold? Valley of the Ancients comes to mind, of course, but there is really nothing there. Hard to see that place as some kind of ancient power center.

Invasions could be the Peshwah invasions, and the civil war could well be the mage war, but where/what is the Valley of the Kings?

Normally I would assume this is a reference to the Valley of the Ancients in the distant past. Maybe the valley was in a more habitable state when Thonia first ruled Blackmoor, or maybe it had been the historic home of ancient kings and still was a center of power. Hence its name. But the Valley of the Ancients was formerly home to a great salt sea, so I'm not sure if all of this would make sense given that fact.

I speculated that this was a location in Thonia, representing perhaps the royal line of Genova.

But it could be a reference to something else.

-Havard

Okay, we don't seem to know, I guess.

I think the Thonia location doesn't work very well. If the "bloodline" of High Thonians is only in power in the "Kngdom" of Blackmoor, when the "Valley of the Kings" stands strong, its hard to see it as someplace outside the kingdom, unless by kingdom we really mean duchy. I guess it could be a kingdom within an empire.

However, either way, we are left with a "kingdom of Blackmoor" ruled by Thonians some time in the distant past, presumably after the conquest of Robert !. Perhaps it was a brief moment of a century or so.