From a strictly chemical standpoint, Epsom Salts is just magnesium and sulfur. Magnesium is a nutrient required by plants, as is sulfur; both in trace (very small)amounts. If there is a deficiency of either then the plant will not strive very well.
I personally don't believe that it is strictly a chemical thing, but would accept that it is an ionic (electromagnetic) thing. I just don't have any proof of it.
Sulfur is used to shift pH toward the acid side and tomatoes seem to like acidic soil according to the experts. Yellowing of leaves is one indicator of chlorosis which may be an indicator in blueberries that the soil is too alkaline.
Other uses of Epsom Salts suggests that there is some unknown or unadmitted effect of it. We have a lot to learn and we can start if we just admit that we can't explain everything by using scientific methods. I'm not poopooing science, just suggesting that we have to think weirdly to understand "weird" things.