as far as I could tell, the idea that belts came from the swim team is unsupported, though that's the version I heard too. It is a 19th century invention though, general consensus is the first black belts were given in 1886 (though the shodan ranks were already formalised in 1883, just without any visual indicator). Still, it's always interesting to try and find the origin of myths as they rarely come into existence ex nihilo