Business logic layer contains all of the business logic. Its responsibility is to validate the business rules of the component and communicating with the Data Access Layer. Business Logic Layer is the class in which we write functions that get data from Presentation Layer and send that data to database through Data Access Layer.
BLL Classes related to what we need on UI.
Entity Classes related to what we need in database.
Sometime we are working with Service oriented Architecture in that case we can't pass entity directly so we need DTO (data transfer object) that we design according to what we need on UI.
Business Logic Layer sends data to Data Access Layer, does not send to Database so Business Logic Layer Design according to presentation layer.
3-tier architecture in C#