Minnesota hasn't been hammered with rains. Only the Midwest. He also indicated the plant seemed to be growing well enough each year and producing blooms. A plant that has a failing root system from water issues may very well bloom as a last ditch effort to reproduce under stressful conditions but it won't generally put out any new growth after that. At least that's been my experience and I'm in the Midwest where we have been forced to deal with heavy rains and severe flooding the past few years. Although over-watering and too much rain can certainly cause issues, I don't believe that's what's going on here.
Photos of the plant and leaves are always best if available however I think Roger Benson described classic iron chlorosis, "The problem that I see is that its leaves are two shades of green. The darkest green areas seem to be nearest the veins within the leaves, as a very definate branching pattern of dark green color is visible. The rest of the leaf surface is a much lighter green."
I think based on the above it's probably safe to skip a drive to the county extension office to test the soil because hydrangea leaves depicting iron chlorosis routinely retain green veins as described by Roger Benson.
Here's a link that helps describe differences in leaves associated with chlorosis-
The above article recommends the use of chelated iron or ferrous iron sulfate for immediate results. The application of either would be a quick fix. I still believe the sulfur is the way to go because it corrects the deficiencies for the long haul so this doesn't happen again next year. I suspect this problem began last year as the plant began establishing itself and maturing. If your leaves green up in a month or so, next year you might want to begin using sulfur on this particular plant starting in April.