How much weight does a blisk save?
Very significant.
A research on blisk cost estimation with funding and data provided by Rolls-Royce states:

Integrally bladed discs, commonly known as blisks, are currently found in axial-flow compressors of gas turbine engines. Fig. 1 shows that blisks require significantly less material because the dead weight from the blade roots, disc lugs, and the disc structure required to support these features, is no longer required. This leads to a weight saving of up to 30% (Rolls-Royce, 2005), permitting higher blade speeds and consequently higher pressure ratios per stage (Steffens, 2001). A blisk compressor therefore requires one third fewer rotor stages to achieve the same total pressure ratio as a conventional design (Steffens, 2001). [emphasis added]

Fig. 1. Blisk weight saving.


Langmaak, Stephan, et al. &quot;An activity-based-parametric hybrid cost model to estimate the unit cost of a novel gas turbine component.&quot; International Journal of Production Economics 142.1 (2013): 74-88. (PDF)