Is Blanketweed Dangerous?
Short answer, yes it can be dangerous to your pond and its habitats.
Blanket weed needs the same nutrients to grow as all your other aquatic plants. Therefore, It can grow very quickly and will easily outperform and outgrow your other marginal or floating plants. Furthermore, killing off any submerged plants due to blocking out any natural light.
Blanket weed will compete with fish for vital oxygen which could lead to your fish dying through lack of oxygen. Furthermore, fish can sometimes swim into blanketweed, get stuck and not be able to free themselves and eventually die