Desperate Block says "Block any damage 5 or more," and Mindless Battlerage deals 5 damage, so Desperate Block should save me. However, I just Bashed an Ogre Bruiser to death (and thus was facing it), had it hit me upon its own death due to Mindless Battlerage, and did not see Desperate Block trigger. I get the impression this has been reported before, but I couldn't find a mention of it for Desperate Block. Edit: Okay, it also ignores Parry and Shield Block, so it's probably everything, and I'm sure it would have been reported by now. I can't find it, so I apologize for redundancy.
Ahh . . . I'd say there's a chance for confusion. Consider: does the Hydra's Four-Headed Bite trigger Blocks? It should. And the card uses the word "target": so even though it doesn't care whether there's a single enemy or twelve, the expectation the player gets is that the characters are "targeted" and have permission to Block. Mindless Battlerage appears to be similar, since the game bills it as some sort of "raging attack" done "in battle." It flattens nearby targets whether there's a single one or twelve. A player would expect it to operate the same as the Four-Headed Bite, with the sole exception that it hits both enemies and allies. I'd discourage allowing cases like this where expectations are trumped by game technicalities. A player would understand that the damage from Spiked Mail wouldn't trigger a Block; a player wouldn't understand that a flailing fist from a dieing Ogre is somehow immune.
The thing that makes you not block it is the fact that it is a triggering attachment tho. The same reason ground effects can't be blocked they aren't attacks they are effects.
Correct, it's not targeting anything and so can't be blocked. It is a technicality though, I'll give you that.