Perhaps this video will provide some answers to your questions:
Today, blisks tend to be machined out of forgings and not cast. Repairs can be made and are made depending on the number, condition and location of the required repair. Blisks are lighter than individual blades slotted into a separate disk. It provides for blade aerodynamic improvements including potential reduction of the number of required blades over that of disk slotted blades. Compressor efficiency is improved because of leakage elimination which occurs with disk slotted blades. Also, bolts and blade retainers are eliminated, less parts, less parts inventory to track and maintain.