We did complete the blackwater line replacement last weekend. I have to say, it really wasn't bad at all. Thanks to all of the suggestions I received here, AND the use of Microtabs Holding Tank
Treatment - there was NO smell. The only problem hoses we had at all were those that went from the tank to the overboard
pump, and there to the thruhull. They had been closed for quite a long time, so they were a bit funky.
I had treated the tank with a Microtab a few weeks ago. When we pumped out, there was NO smell at all, and the lines really weren't bad. We stuffed a couple of paper towels in the end, put a ziplock with zip tie over the end, and that was really it.
I can say, even at $5 a pop, we'll continue to use the Microtabs.