I'm not quite ready to declare specific victory, too many unknowns, but since I installed the carbon filter on the blueberry irrigation there appears to be a marked improvement in my plants, darker leaf color and way more berries this year than last (I got 3 if I remember correctly). Shortly after installing the filter I did hit it with beneficial-fungi, and I did fertilize. I know tried adding fungi last year and the plants still struggled. I don't have specific memory of fertilizing but it is a sure bet that I did. I'm pretty sure I sulfered as well last year, so maybe the sulfur is just now kicking in. There was an iron application as well this year.
So I don't think it was just the filter, but filter-fungi-fert. I appear to be doing something right, for a change.
Next up is put everything on the filter, then I want to add an injection system for vinegar and other ferts.