The last book in the original Goosebumps canon.
This is Goosebumps #62: Monster Blood 4
Evan Ross (yet again) is staying with his Aunt Dee and cousin Kermit and it more or less follows the same beats as Monster Blood 3 until Andrea brought what was assumed to be another can of Monster Blood. Instead the contents were blue instead of Green. Instead of growing, it turns into an otherwise innocent blue slug until it drinks water. It begins to multiply and become aggressive and seek out more water...
Honestly, because of both the explanation for the blue Monster Blood as well as the twist ending, this book more or less made the original Goosebumps canon go out with a whimper.
©Scholastic/RL Stine