The Blob was a deranged Changeling.The blob was one of the hundred infant Changelings sent out to explore the Alpha Quadrant. When its vessel crashed on Earth, it was exposed to some substance in the air or soil that induced psychosis and an increase in mass (similar to Odo's experience with a psychotropic gas on L-S VI).
The Blob is a mutated form of The Thing.After the disaster at Outpost 31, the Thing was contained by the government. After this they launched it into space as an experiment, and it mutated into the Blob. Now it still has it's shape shifting powers (tentacles), it's even possible that it can assimilate (as seen with Vicky in the car), but it is now much more blobby than The Thing, which was always very solid for a shapeshifter. However, it has now lost it's weakness to fire, and sees no reason to go with the stealthy root, since it is pretty much unstoppable. It still retains it's weakness to cold, however.It's possible that some of the town members were assimilated by the Blob/Thing, even after the monster was seemingly destroyed.