I've always wondered about this, and I think it would be nice to get it in the chat log. While sometimes it is obvious because your damage always gets cut to a certain amount, sometimes it isn't as easy to tell.
It would be nice if this could be implemented, but without adding another line in the log. It should ideally be on the same line, like when you get a resist-proc from a resist-something trait, or like counter attacks are shown.
Example:
Shield block/guard! The Mandragora hits Mirage for 1 points of damage.
Or:
The Mandragora's attack is guarded/blocked by Mirage. Mirage takes 1 points of damage.