The blobs technically don't have a personality due to the lack of a soul inside them. However, they are programmed to kill and collect souls. This means that they are perfectly fine with killing people even if they are innocent. Also, it was shown that they can combine with each other in order to become more powerful and larger. They are attracted to human souls which is how they can hunt down other humans, however, they are not attracted to grey souls.
Soul Harpoon - They can pierce the body and pick up the human soul. The blob has a pick used as a harpoon for killing flesh made organisms (humans).
In Do or Die, it was seen that a blob can move at the speed of a regular human child.
It was seen that blobs can move on floors and ceilings.
Blobs can take people's appearance making them able to use it as a cloak.
It was shown in Do or Die that material objects (Like Alphys' anime case) can hurt them.
At least 23 people perished during the battle for the city. But It was said that the number could be bigger due to the fact that some blobs couldn't join Betty and give her the souls collected.