Un-blocking the Yubikey PIN seems a bit un-documented. Here is how, bear in mind, the password (-P option) is 8 characters max, and if you type anything else in it will tell you the PIN can be max 8 characters, not the password. Totally stupid.
% /usr/local/bin/yubico-piv-tool -P <8 chr password> -N <PIS> -a unblock-pinPIS = Personal Identification String (The manual call this a PIN, which is wrong). This is the string you type in every time you use the key.