This seems a bit odd. How could such inferior 'aftermarket' blades possibly be certified for use?
All it really takes is for a DER to sign off that the parts are "functionally equivalent".
This may be a complete red herring for this accident, but maybe not. At best, PMA parts have been something of a dirty secret of the industry - for but many people (me included, and I was a DER) it's been an accident waiting to happen. The engine manufacturer (and airframe manufacturer) are required to perform exhaustive testing and analysis to prove that the various parts are airworthy before they can be certified. But under PMA, someone can 'reverse engineer' the part and make a Chinese copy, get a DER to sign off that it's 'functionally equivalent', and it becomes a perfectly legal, acceptable replacement part without having to perform all that troublesome cert testing. Since the PMA doesn't have all the overhead of testing, etc., they can significantly undercut the OEM on price.
For a long time, PMA was pretty much limited to 'consumables' - things like filters, seals, etc. Although even that can go wrong - back 10 or 15 years ago there was a big mess on the 737NG when a batch of PMA fuel filters started coming apart in service and thoroughly contaminating the fuel control system they were supposed to protect (I'm thinking there was an AD to get them out of service).
Now days it's extended to internal engine components - such as turbine blades (part of my DER recurrent training several years ago was how to certify PMA turbine blades
). A while back, as their market share sank, Pratt and Whitney started making PMA turbine blades for the CFM. Now, I suspect P&W PMA blades are fine, other PMA blade manufactures not so much