While Blake2b is capable to produce digests of any size from 1 to 64 bytes, It's default mode is 64 bytes (512 bits) though.
Some libraries implement just default digest size.
* OpenSSL supports blake2s-256 and blake2b-512 (no blake2b-256)
* gcrypt supports both blake2b-256 and blake2b-512
* nss supports none of blake2b
* Botan - any digest size
* Java: https://github.com/alphazero/Blake2b - any digest size
* Go-lang: https://godoc.org/golang.org/x/crypto/blake2b - both blake2b-256 and blake2b-512
* Rust: https://docs.rs/blake2/0.8.1/blake2/ - any digest size, 512 by default
* JS: https://github.com/dcposch/blakejs - any digest size, 512 by default
Also various libraries based on openssl will provide just blake2b-512.
So it looks to be a preferable choice over blake2b-256
