Go read James 315's article on how the "blue donut" is a myth, and then come back here. Done? Good.
Now, he's not entirely wrong. He brings up some good points - nullsec has never seen an era of complete peace, ever - but it quickly descends into fallacious assertions and elitism. The blue donut is very real. It's just not the one described by James 315, as that concept is a strawman and a ridiculously inaccurate one at that. Rather, the blue donut is the principle that nullsec would rather slapfight or band together to trash highsec than actually fight each other. "Shoot blues" may be the phrase of the day, but "shoot structures" isn't.
There are two kinds of people that point to the blue donut, as mentioned by James 315: nullsec line members and highsec carebears. Both groups tend to lace their statements with hyperbolic nonsense (RMT moongoo is the stupidest thing I've ever heard), but inside each is a nugget of truth. So let's go!
Fundamental Problems With Sov
Nullsec tends to complain of the blue donut because they are sandwiched between allies, unable to find a decent fight within deployment range. Some alliances circumvent this with some seriously impressive logistics groundwork, but most seem to sit in their corner of space and only bring the big fleets to their own neighborhood. Even when they do, sov changing hands is not on the menu; shoot bad guys, but not their structures!
There are two reasons for this. The first is that cross-region traveling is, frankly, a pain in the ass. Without a decent highway through highsec, going from one end of nullsec to the other takes ages. Picking fights in regions beyond the ring of allies is troublesome, as it involves some incredible logistics work and long travel times. People want to fight, not watch their ship in warp for an hour before trying to fight an enemy that inevitably flees.
So yes, technically you can find an enemy in eastern nullsec to fight. It is, however, so far away and so boring that it's not generally worth it unless you have a beastly support crew willing to keep you fueled, armed, and shipped up.
The second is - and we all know this - that sov is garbage right now. Top-down sov encourages static structure-shoot play and the manipulation of reinforcement timers and timezones. That's incredibly boring, especially since there's no real way to engage in actual, real-life-styled warfare. The alliances are so rich and the economic means so safe that effectively waging a war of materiel attrition is near impossible on a large scale. Rather, most wars are won via morale.
James 315 brings up the dissolution of coalitions and alliances as proof that nullsec isn't part of the blue donut. That's patently false. The major dissolutions occur because of morale breaking, high-level betrayal, or good, old-fashioned internal politicking. Alliances and coalitions do disband based on external pressure, but the major wars of nullsec are fought less on the actual battlefield and more in dark alleyways, intel briefings, and smoke-filled political halls. With sov being so static, the balance of power has shifted from total warfare to corporate espionage, as nobody wants to show up to shoot structures with ridiculous HP totals. This is why spymasters and politicians like The Mittani are so effective.
Trash on Highsec
The other side of the coin is the highsec carebear. These poor creatures are the subject of much bittervet discussion, despite not having much representation in media and politics. Perhaps it's just their gentle, caring nature.
The blue donut concept appeals to highsec because there is a very real section of the EVE populace that looks down on them, and most of those people come from nullsec. James 315's extortion scam, for example, regularly pisses off miners. It leads to some hilarious ragemails (as seen on his site), but it also adds to the concern that nullsec's goal is to trash highsec and the people that play there. The "messiah of highsec" schtick, along with proposed changes that sound more like EVE as it was instead of EVE as it should be, pisses people off.
There are also the null and lowsec-run events such as Hulkageddon and Burn Jita, which specifically target the highsec demographic (miners and haulers, mostly) and draw in nullsec players from disparate, often-warring alliances to annihilate their common enemy: the carebear. When faced with such an organized resistance to their way of life from nullsec, it's understandable that highsec puts faith in the blue donut concept. They've directly seen the actions and results of nullsec-run events aimed at uniting against highsec.
Perception is Everything
EVE is a game of perception. While the literal mechanics of the blue donut (everyone in nullsec sets each other blue) are not - and never will be - a part of the game, the results are similar to what would happen if they were. Nullsec alliances seem more concerned with slapfights and espionage than all-out conquest, with some exceptions, and highsec sees regular "nullsec invasion" events. These are the characteristics of the perception of the "blue donut": a ring of alliances too afraid, lazy, or annoyed to truly fight one another.
When you shift the definition from the commonly-accepted perception to a literalist strawman, of course there is no blue donut. There never could be. But the blue donut isn't quite so black and white. Much like EVE itself, the concept is somewhere in-between; a gray area of boredom, pretend battles, mock war, and null elitism.