The first two seconds are many things. Though there's a lot of good in them, they can be atrocious, interminable, poorly written, abysmally acted, embarrasingly scored. But they were never bland.
The remaining five seasons were generally of better quality, but they were far more prone to blandness. So, while my least favourite TNG episode - and worst hour of Trek in all time - would be "Up the Long Ladder", it's too silly and campy to qualify for the 'most bland' moniker. I'd never watch that "When the Bough Breaks", "Haven", or "Angel One", ever again, but none are bland.
So what's the most bland episode of all time? Hmm. I'm not sure, really. The bland episodes kind of blend together, unlike the terrible early episodes that are very distinctive.
I mean, even bland episodes I can think of have redeeming qualities - like "Suspicions", a painfully by-the-numbers murder mystery, has the novelty of a non-pathetic Ferengi character. I've probably simply forgotten this utterly bland, pointless episode I wouldn't watch...
Though I like "Allegiance", it's fun. I kind of like "Bloodlines", but mainly because I'm fond of "The Battle" (though it's a same Frank Corsentino couldn't reprise his role of DaiMon Bok for the latter showing).