Parkerizing need a rough, clean surface to adhere. Blueing is billions of microscopic pits in the metal; its a controlled rusting process. The trapped oil in the surface of a blueing job will screw it up. Just bead blast the parts, try to avoid handling them at all. Use stainless wire bent as part hangers to avoid touching with your fingers or an oily glove. I cant tell you the number of times I had an otherwise perfect parkerizing job screwed up by an oily fingerprint.
Home refinishing isnt without its problems, and in my opinion make friends with a local shop that does this kind of work. Offer to bring him additional work of the same type to bring the price-per-part down. Maybe a friend needs an 870 parkerized at the same time.
Oh yeah, I've home parked a few tings already, including my project. I'm going over some buttons that were previously blued and I was curious to the exact reason why the blue must be taken off. I have my solution raring to go