Beyond human comprehension hyper intelligent, but it doesn’t take much notice of humanity.
The blob itself is a hyperdimensional being, all blob cells are part of the same organism. It’s less that it’s a hive mind and more that it’s a single organism that is distributed throughout the multiverse, Humans aren’t capable of interacting with it in terms of organism-to-organism, humans just interact with its individual cells.
The actual Cataclysm event was a planetary-scale weakening of the normal barriers between our dimension and the foreign dimensions the blob, triffids, mi-go, Mycus, etc exist on. Effectively during the course of human experimentation into other dimensions, they attracted the attention of the actual organism composed of the Blob, and it decided to colonize our universe. Just a moment later and it was done, the Blob had permeated the entire surface of the planet, and in doing so it opened the path for other opportunistic dimensional aliens to use their own means to travel to Earth as well. From that point on, the majority of life on earth was infected with the Blob and the zombie apocalypse was inevitable. As if that weren’t bad enough, the weakening of the dimensional barriers allowed innumerable other aliens to gain entrance to our dimension, rampaging across the planet and either establishing colonies (the Triffids and Mycus) or simply raiding the planet for various resources (various Nether creatures and mi-gos). Over the next several days of utter chaos, the vast majority of humanity was wiped out, and the Blob, Triffids and Mycus solidified their presence.
Extra-dimensional, don’t know, don’t know, don’t know, don’t know.
Slime mutations have nothing to do with blob, other than the fact that it’s the blob causing all mutations.
All mutations are based on the blob editing DNA, usually by copying foreign DNA into the host organism’s cells. this is the basis for “flavored” mutagens, you stimulate the blob to trigger mutations and simultaneously flood it with a bunch of foreign DNA of a particular type, so it ends up being much more likely to incorporate traits found in that source DNA instead of from elsewhere.