RDBMS CONCEPTS

The use of keys are instrumental in database management. Primary keys  allow for distinct records and foreign keys tie records together to  create unique relationships between two or more entities and/or tables.  In Blockchain, we know that a hash is the equivalent to a key - and  makes the chain secure and indisputable (or so we think). Why is it then  that with an RDBMS, a primary key and foreign key can still create  redundancy, thereby causing data anomalies? Can the same be said about  blockchain?