A Blisk is a bladed-disk, all one monocoque component.
So the blades do not have to be inserted and secured, and they are not removable. No nuts and bolts either.
The downside is that any serious damage to one blade, and the whole blisk must be replaced. The upside, I have heard, is that they save weight. Wikipedia says "efficiency improvements of up to 8% are possible." but I'm not interested in aerodynamic efficiency right now. I'm interested in weight savings.
So the question is, how much weight can you save? Is it really significant? Interested in compressor and turbine blisks and I would not be surprised if the answer varies between them.