My mother-in-law Dick was our very own “America’s Test Kitchen” well before Christopher Kimball. She was always experimenting, and couldn’t contain her enthusiasm over a new find. She called one day bubbling over with excitement. She told me about this light, fluffy blueberry coffee cake. She would mail me the recipe, and she just knew I’d like it. I’d heard she’d been looking for a blueberry coffee cake. Perhaps it was because her signature coffee cake (see Dick’s Sour Cream Coffee Cake) was chock full of nuts, which many people avoid. Or maybe it was just blueberry season. I’d had a favorite blueberry cake, but this replaced it in no time. All too often,…