Well....Magnesium sulfate(Epsom Salt)is not considered an importent nutrient for flower bud set/production (its primary usage/need in the plant is for chlorophyll production).With that said, if you like how it makes your plants look and bloom, DO NOT change...plants don't read the books:-)>
BUT...it is a common, underutilized nutrient in the garden that can really help keep your plants "GREEN". From the standpoint of a healthier,greener plant...you could say that it was helping induce flowers(from an indirect standpoint it helped "encourage blooming").
From a minor nutrient standpoint, Hydrangea macro. does love Iron and I do think the Epsom salt would be beneficial also. A quick remedy to see how they react is a foliar application....1.5-2 Tbsp.s Epsom Salts per gallon of water. spray to drip in the EARLY MORNING hours (1/4 Tsp of dish soap...just a little bit... as a wetting agent)

Epsom's salts (magnesium sulphate) can be beneficial to plants that have a high magnesium demand - tomatoes and roses come to mind immediately - and it does promote the development of chlorophyl as schmoo mentions, but there is nothing in this amendment that improves flowering ability. And the routine use of an amendment like this can lead to an overabundance of specific elements that could produce counterproductive results unless you perform regular soil tests.

In a PNW climate, partial shade is desirable for most hydrangeas - morning sun and afternoon shade is ideal. Too much shade can result in impaired bloom and too much sun will result in daily wilting in summer and bleaching of the foliage. It often takes a season or so for new shrubs to settle in, establish a good root system and perform as described. I wouldn't be overly worried about LinR's lack of blooms unless it suffered any winter dieback (possible in our bizarre weather last winter) or it is receiving excessive fertilization. Both of these can affect bloom potential. Most likely it just needs additional time.

And I'd avoid applying any sort of amendments or fertilizing unnecessarily. Test you soil first and apply only as the test indicates deficiencies are present. FWIW, a regular mulching with compost or other quality organic matter will reduce the need for pretty much any supplemental amending or fertilizing and is most beneficial for the soil organisms.