A typical concrete block (CMU) assembly rates around STC 45. That is already good for most rooms; upgrades matter for theaters, studios and apartments.
Most effective upgrade
Fill the cores with grout or sand, seal the surface with block filler paint or plaster, or add a furred drywall layer on isolation clips.
Acoustic panels on the surface add only 1–3 STC points. They are for the sound inside the room. See STC vs NRC and flanking paths.