There's a huge difference between cryptographic and non-cryptographic. Note that blake2 has various length, whereas SeaHash is fixed to 64-bit (although I suppose it's not to hard to make a version with bigger length), and thus naturally collisions will happen.

If you need fingerprints, don't use SeaHash, but if you are looking to insert into e.g. a hash table, you shouldn't use BLAKE2. It's awfully slow for that.