The blog author could allow commenters to publish their keys and signatures. If the author later publishes their comment, the reader can verify 1. it has not been edited (a hash would also suffice for this) and 2. it was signed with the commenter's published key (presumably by the commenter).
Sloppy terminology. Apologies.
For example, here's the 64-bit signature for the comment above:
e3b2b607891e364b0c80df9fe926a5aed125d8f4a0f2f4c58b58b87b92884e634bb614f13cad372f074158c035eb382535b6d33d9ba10d7d59843a17dee5720e
Here's the 32-bit public key:
bf9fae774061bb6f5deea9380ccdf11263071bc256c1bd0947572f0a3dcc3806
The blog author could allow commenters to publish their keys and signatures. If the author later publishes their comment, the reader can verify 1. it has not been edited (a hash would also suffice for this) and 2. it was signed with the commenter's published key (presumably by the commenter).