The "blame" is gonna go back and forth. Boeing is going to say that they were never "required" by a formal FAA order to implement the measures recommended by the NTSB and that they "fixed it more than adequately" by warning people to test TOWS more often.
I believe there's still concern in some NTSB quarters that some of their recommendations are not taken seriously enough by the FAA; one example being those after the post 9/11 Airbus vortex wake/fin failure incident..