I thought it was pretty clear the Fortress is (was, since it is apparently down now per TRO3150?) a device or devices. Obviously some kind of hyperspace device that prevents jumps through it... at least for those who do not have means to by-pass it (meaning everyone but the Republic's forces). Speaking of by-passing it, i though it might be as simple as jumping next to the edge, moving through space by conventional propulsion, and then jumping again. Considering the edge is likely in middle of nowhere, it is unlikely anyone would realize to try this, especially since it is so unintuitive in the era of JumpShips. Not to mention the time it takes and somehow determining where the edge is without knowing the limits of the generator.
Logically, the Blackout is also caused by a similar device but tuned to the hyperspace comms rather than jumping. Though the HPG network was supposedly brought down by viruses, sabotage, and other such actions, i would assume that is merely a result of rumor and cover stories.
If any mundane thing was the cause for the Blackout, it would not have lasted so long across so wide space. The richest nations could have funded replacement stations (assuming old ones could not be fixed), in such situation the Blackout could have covered everything but, for example, the FedSuns and the Republic.
Tucker Harwell fixed one HPG, temporarily, and the fix didn't work again apparently. Indeed, his talk about hyperspace frequencies is telling, there's something wrong with "hyperspace" itself. (We must remember the BTU doesn't have "hyperspace" like Star Wars has, rather it is merely a term for mathematical hyperspace. The jump drive is effectively a teleport or a local wormhole generator, it doesn't go through another medium like Star Wars hyperdrive.)
I would assume the Fortress and the Blackout generators are based on the same tech like the jump drive and hyper-pulse generators are.
I think the Blackout device may have been an offensive device for the Republic use post-Fortress protocol, to disrupt enemy communications so that they could re-conquer lost territories or something like that. But the fate lead their use in reverse.
It is highly unlikely the Blackout and Fortress generators are simply one device each. The Republic in 3145 covers an asymmetrical area on a map. A Fortress-field generator presumably generates a bubble, as most fields (magnetic, gravitation, etc.) tend to be. Were there only one powerful device on Terra generating a bubble, it would likely* cover some areas that the Draconis Combine rules at the moment.
This would also explain why the entire Republic was not covered, the cost of such devices is likely massive.
*Remember 2D map of space is inherently inaccurate, there are stars "above" and "below" the zero point, something a 2D map cannot represent.
All in all, the Fortress and the Blackout are not so big mysteries, presumably they're merely extensions of Kearny-Fuchida theories, combined with advanced devices capable of manipulating the hyperspace beyond just moving through it. I mean, there's a lot people in BTU do not know about the hyperspace, this much is clear. The Blakists and some others have created super-jump drives, something thought impossible previously. And given the Succession Wars, the research into hyperspace was stagnated for a long time.
The far bigger mystery is who... and we have only hints and guesses about that.