While "blub" doesn't have a specific meaning on its own, it's the base of various words like:
* Blubber (the thick fat of whales and other marine mammals)
* Blubbery (describes something that is fat or fleshy)
* Blub (a sound made by crying or sobbing)
The origin of "blub" itself is likely imitative, representing the sound of something thick and soft being struck or compressed.