If the pH and balance of excess EDTA in bleach and fix are not correct, then some darkening of the dmin can be observed. If the sulfite level is off, the same can happen. All of these are from different causes.
1. Bad bleach = Retained silver and retained yellow filter layer.
2. Bad fix = Retained silver halide and gradual darkening of the film.
3. High pH = Dark film due to developer stain from continued development - this cannot be fixed!
4. Low excess EDTA = Dark film due to retained Iron salts - this cannot be fixed!
5. Low excess Sulfite = Dark film due to developer stain - this cannot be fixed!
PE
Thanks for this very useful info. This gives me a few pointers to troubleshoot my process.
I am using a JOBO processor and fresh C-41 flexicolor from Kodak, different kinds of film (Kodak and Fuji). Use pre-soak, dev, bleach, rinse, fix, rinse, stabilize. Dev three films at once in 600 mls.
All films, not just exposed areas, appear opaque (no gradual darkening), Kodak branded film dark brown, Fuji branded film dark purple; makes it impossible to scan. Same batch film at local photo lab give nice and clear negs

.
I re-bleached the negatives, but no luck after 4 h. Made up new chemicals from scratch in supposedly DI water, performed bleach steps and fix steps for 30 mins each instead of the recommended 6.5 - same thing - opaqueness, always worse with Fuji than Kodak film. pH seems fine. According to municipal water source iron is low. I don't have access to an AAS, so hard to evaluate actual trace iron.
Should I try adding additional EDTA and/or sulfite to the bleach? What concentrations would you recommend? Should I try making everything with DI water next?
Thanks for your help.