token of authenticity they can rely on

Rely on for what? In cryptography, authenticity refers to the source or origin of some data; in other words, the identity of the creator.[1] Likewise for official seals--the seal attests to the identity of one of the parties (either one of the principles, or an attester).

Your usage regarding seals seems correct. It's the analogy with the blockchain that is confusing. It's difficult to tell without more detail. If you broke things down you'd probably end up also using terms like integrity and non-repudiation, which are quite distinct.

[1] For example, an excerpt from the Handbook of Applied Cryptography (http://cacr.uwaterloo.ca/hac/about/chap1.pdf):

  Once the TTP creates a certificate for a party, the
  trust that all other entities have in the authenticity of
  the TTP’s public key can be used transitively
  to gain trust in the authenticity of that party’s public
  key, through acquisition and verification of the
  certificate.

  -- Chapter 1, Section 1.11.3.