According to New Scientist, there are several varieties of venomous lizards, but the presence or absence of a blue tail isn't enough to determine which lizards are poisonous and which ones are not. In fact, as recently as 2005, there were thought to only be two types of venomous lizard.
New Scientist states that up until 2005, only Gila monsters and Mexican bearded lizards, both of which can have bluish tails, were believed to be venomous. However, based on research that was published at that time, it's believed that most monitor lizards and iguanas also produce venom. Before 2005, scientists erroneously blamed inflamed or deadly bites from iguanas and monitor lizards, including Komodo dragons, on the lizard's mouth bacteria instead.
Though not an issue for humans, American five-lined Skinks are toxic to pets and other animals. They are one of the most common lizards on the east coat of the United States, and young five-lined skinks do have a noticeable blue tail. When ingested, these lizards can case Peripheral Vestibular Disease in animals. Though debilitating to the animal, it is usually not a lasting condition, but a vet consultation may be merited if a pet has come into contact with one.