Superphosphate does encourage blooming, but it is seldom required as phosphorus moves through soil extremely slowly and is most often present in sufficient concentrations. Too much phosphate creates a toxicity in a number of plants, so I'd be reluctant to overuse this product. Also, superphosphate is manufactured from a chemical reaction of rock phosphate and sulfuric acid and therefore tends to "sweeten" soils. Mophead hydrangeas treated to superphosphate will tend to bloom in the pinks and purples rather than blue.

I am letting in my personal bias here, but I tend to shy away from any kind of chemical-based or synthetic fertilizer or soil amendment. They are harmful to soil organisms, too easy to overdo, leach into groundwater too freely and even moderate applications can create nutrient imbalances, salts build-up and potential root burning (superphosphate is very inclined in this last regard). Applying a complete blended organic fertilizer can supply needed nutrients, however most plants - hydrangeas included - are well served by just periodic mulching with a quality compost, nothing more.

Lack of flowering with hydrangeas is generally not associated with insufficient fertilizing. Rather, it is most often caused by cold weather that damages flower buds, improper pruning or unnecessary fertilizing with a high nitrogen fertilizer.

Thanks for your info. I would prefer more natural sources of fertilizer as well. I just added some dried blood to many of my flowerbeds, including the hydrangeas, because when I tested the soil, it was deficient in nitrogen.
As far as color, I actually prefer the pinks and purples, so that does not worry me.

As far as blooming, I am going to try one of the tenting methods to protect the new buds over the winter, or the method where you put something heavy on the plant and cover it with cardboard and mulch. My biggest plant is far too bid to cover with a pot, like I tried last year.

You are fortunate to live in a climate where you can easily grow hydrangeas and not lose the old wood! I am envious.

JR