Why replacement turbine blades are not simple copies

Could a navy manufacture replacement turbine blades locally after foreign supplies were cut off?

Eventually, but not quickly: reproducing them requires specialized equipment, materials, and experience that take years to establish.

Possessing a sample does not create the industrial process needed to reproduce it reliably. Establishing materials, precision manufacturing, quality control, and skilled labor would take years, making dependence on a future enemy's components a serious wartime vulnerability.