Whenever people discuss mammalian-type merfolk, I usually see them be modeled after cetaceans like dolphins and whales. This is perfectly serviceable, but there is one piece of anatomy that I cannot figure out how to apply to a mermaid: the Blowhole.
As to why a mermaid would have a blowhole, the reasons I can think of mostly revolve around sleeping and energy conservation. Whales sleep in short naps close to the surface of the water, so they can take breaths if they need to. As for energy conservation, I think it would take more energy to tread water and keep a mermaid's whole head out of water than to just pop up and use the blowhole.
Hence my question. If a cetacean-based mermaid had a blowhole, where could it go?