(...) The defining characteristic of a blobber is that you have multiple characters fused into a blob that moves together. Nothing else applies here, and if you would argue otherwise, then you'd be wrong.
You can, of course, define words however you want. But when we are talking about games, a game's "type" is defined by its gameplay. So, for instance, genres such as "adventure", "platformer" or "strategy" are defined by specific aspects of the gameplay. A platformer involves making well timed jumps, exploring a stage and avoiding or defeating enemies, etc. Rather than genres, we might have certain words that describe some important feature that cuts across various aspects of the game as well. For instance, while a game being "multiplayer" says nothing about its gameplay; it does indicate that various aspects of the game will be geared towards supporting multiplayer. Other such words could be "story heavy" or "story driven", "sandbox", etc. I have argued that RPG falls in this category of words, not defining a gameplay genre by itself.
Anyway, my point is that if the definition of a blobber is about whether you have many people in your party which you control as a single person or not, then it doesn't really tell you much at all about the game, either as a genre or as a whole. I mean, there being multiple people in your party is usually significant for gameplay, but technically you could have the same gameplay with just one person who can take multiple moves. As such, I think it is more appropriate to use blobber to define a specific type of gameplay...
Is Ultima Underworld a blobber? Is Daggerfall? (...)
Again, define words how you want. But I think the best use for the word "blobber" would be to describe games where you have combat and exploration reminiscent of how D&D used to be played. In particular, exploration should rely on player made maps (just like there used to be a player that was responsible for mapping the dungeon) and combat be similar as well (turn based combat works well here, but I wouldn't put it as a requirement, action combat of real time blobbers though would be right out).