I'm not terribly familiar with blackbox, but based on the blackbox Readme it seems that this critique from Vault's website [1] addresses why HashiCorp thinks Vault is a better solution.
> Chef, Puppet, etc. all solve this in a similar way: single-key encrypted storage. Chef has encrypted data bags, Puppet has encrypted Hiera, an so on. The encrypted data is always one secret (a password, a key, etc.) away from being decrypted, and this secret is generally not well protected since in an elastic environment, every server needs to somehow get this secret to decrypt the data. Additionally, access to the encrypted data isn't always logged, so if there is an intrusion, it isn't clear what data has been accessed and by who.
https://github.com/StackExchange/blackbox