Explain difference between Blackboard and Database.

In the Blackboard pattern, the programs that work on the data sources stored in the blackboard, are themselves also inside the Blackboard app.

In a database, programs that work on the data sources originate from external sources.
A database is a generalization of the Blackboard architecture.

blackboard: a collection of knowledge sources, and a control component.

Generally speaking Blackboard and Database are similar. Both store and manage central data which is accessed and modified by other components.
I would say Blackboard is more specialized version of Database. It has a list of possible solutions and controlData and defines only two methods: inspect and update.
Database is much richer implementation of a central storage which in most cases support relations between tables of data and can accept complex queries.
One can make a Database work as a Blackboard but regular Blackboard implementation usually doesn't suffice as a Database replacement.

mostly correct. Architectural styles differentiate Blackboards (active, pushing) vs. Repositories (passive, client polling) and Database can be both. Database is not in the vacabulary of this style.