Where it is not
Blue Lagoon is sold across Unguja as a snorkelling stop, and most listings place it in the south-west, near Kizimkazi. That appears to be wrong. The specialist snorkelling reference that describes it puts it on the east coast, on the Michamvi peninsula between Pingwe and Dongwe — the opposite side of the island, and a different day out entirely.
This matters practically. A traveller booking a beach on the east coast and a Blue Lagoon trip as separate excursions on separate days may be booking two things twenty minutes apart.
What it is, as far as anyone says
A small snorkelling spot in the reef flats. The same reference describes it as a small lagoon dug in the reef flats of the east coast, which if accurate means it is not a natural feature at all. That is a significant claim and no second source could be found for it, so it is flagged here rather than stated.
Beyond that, very little. The name appears on no Zanzibar government list, in no conservation-area document and in no gazetteer. The only mapped feature on Unguja called Blue Lagoon is a seafood restaurant on the Michamvi road. Reports of the snorkelling — clownfish, surgeonfish, butterflyfish, a high density of starfish — come only from excursion listings, and it is commonly sold bundled with a “starfish” stop and a visit to the rock restaurant on the same coast.
The honest summary: this is a boat-trip stop name rather than a place with anything authoritative behind it, and the pages selling it do not agree on which side of the island it is on.
A name that travels
There is more than one Blue Lagoon on the Tanzanian coast, which adds to the confusion. Mafia District Council lists a Blue Lagoon among Mafia‘s own attractions, and the tidal channel at the southern end of Juani Island there is sold under the same name. They are several hundred kilometres apart. An itinerary naming Blue Lagoon without naming the island is not telling you where you are going.
What to do with that
If it is offered as part of an east coast boat day and the price is the price of a boat day, it is a reef stop like other reef stops and there is no reason to avoid it. If it is being sold as a destination in its own right, worth a transfer across the island, the evidence that it is one does not exist.
The reefs that are documented on this coast — Mnemba in the north-east, Chumbe and the Stone Town islands in the west, Misali off Pemba — are covered in the snorkelling guide, which also explains why protected sites look different in the water from unprotected ones.
Common questions about Blue Lagoon
Where is Blue Lagoon on Zanzibar?
Most listings say near Kizimkazi in the south-west; the one specialist source that describes it puts it on the Michamvi peninsula on the east coast, between Pingwe and Dongwe. Confirm which coast before booking a transfer.
Is it a natural lagoon?
Possibly not. The source that locates it describes it as a lagoon dug in the reef flats. No second source could be found either way, so the question is open.
Is there an official description of it?
No. It appears in no Zanzibar government list, no conservation-area document and no gazetteer. The only mapped feature by that name on the island is a restaurant.
Is this the Blue Lagoon on Mafia?
No. Mafia District Council lists a Blue Lagoon of its own, and a tidal channel at the south end of Juani Island is sold under the name. They are unrelated places several hundred kilometres apart.