Vitalik suggested using default algorithms as it makes it easier to code and tinker. Also, some libraries may not support digest sizes.
Also, Justin reminded that we can consider blake as a placeholder for now and not worry too much. The reason is that a STARK-friendly hash function (e.g. AES-based) is possibly preferable to blake.
blake2bis 1.5 times faster thanblake2s with 64-bit CPUs.