As I understand it, the BLAKE family is built on a tweaked version of the ChaCha cipher, and security is proven under the assumption that this primitive behaves

As I understand it, the BLAKE family is built on a tweaked version of the ChaCha cipher, and security is proven under the assumption that this primitive behaves like a random function. Arguably we shouldn't be too concerned that blake3 is new, since ChaCha/Salsa are old primitives whose security has held up well. While the sponge framework is popular, it's rather inflexible, e.g. its sequential structure permits limited parallelism. There are more general sound hashing frameworks that still offer provable security, such as this one which blake3's analysis is based on: https://www.cs.ru.nl/~bmennink/pubs/18fsenew.pdf https://www.cs.ru.nl/~bmennink/pubs/18fsenew.pdf