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 its 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 its weakness to fire, and sees no reason to go with the stealthy root, since it is pretty much unstoppable. It still retains its 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. The Blob absorbs the knowledge and memories of the organisms it absorbs.
The Blob in some cases seems to come off as rather malicious as the WMG's Alternative Character Interpretation has pointed out, which seems strange for an entity that basically stalks and absorbs it's prey. Considering the above mentioned The Thing absorbs memories of characters it's mimicking (allowing it to talk English), The Blob may itself also do the same thing to it's victims, which accounts for why it's intelligent when it comes to certain times it attacks.