A Blockcerts is a digital document that contains all of the certificate information; however, only a portion of the Blockcerts is kept in the blockchain.
The document's digital fingerprint (hash) is recorded on the blockchain. In this manner, the document holder can compute the digital fingerprint and compare it to what is stored on the blockchain. This is one of the verification processes that occurs when the Blockcerts file is opened in Bestr or imported into Blockcerts Universal Verifier.
The blockchain contains no personal information. The content of the certificate cannot be deduced from what is written on the blockchain. Only the integrity of the relevant certificate can be checked.
Only the certificate's owner can choose to divulge it - and so share its content - with a third party, such as a potential employer.