Pending blockchain registration refers to the time it takes for the certificate to be registered on the blockchain.
There isn't anything for you to do and the registration will take place automatically. This generally takes 8-12 hours depending on the number of transactions taking place at the time.
Every time a change is made to the certificate, a new version of the certificate is recorded on the blockchain.
This means that while your certificate may have previously been "blockchain registered" if you may any changes to it, you will once again see "pending registration" appear on the certificate.