blake2b length extension attack approach
BLAKE2 is not open-source. BLAKE2 is a specification.

The state is shuffled one last time after having set the final bit, ensuring that a small change, even a single bit, yields a completely different output, that cannot be reverted.

A length extension attack means that knowing M and Hk(M), you may be able to compute Hk(M||E) without knowing the key k.

But with BLAKE2, in order to compute Hk(M) before the finalization, you would need to know the key k. If you somehow know the secret key, you can already forge tags for arbitrary messages.