The maples where I live are too far along now for harvest, but higher up in the Cascade foothills I found plenty that had just blossomed the other day. You want to get the racemes just as they emerge from the protective red sheath that guards them and the unfurling leaves. At that point the racemes will be compact and tightly clustered; as they blossom, the flower-clusters become large, elongated (several inches or more), and some of the older flowers will have cottony material inside. The newly emerged racemes are easier to work with and make a daintier presentation.
The taste of bigleaf maple blossoms is subtle: slightly nutty with a hint of sweetness. I've used them in the past to make pesto. The most common use is for fritters. My recipe is adapted from Poppy chef Jerry Traunfeld's, which can be found in Jennifer Hahn's excellent wild food resource, Pacific Feast. I used less water for a slightly thicker batter. Even so, this batter is very tempura-like. It's thin, drippy, and puffs up around the blossom upon hitting the hot oil. This makes for a light, chewy, beignet-like fritter that's perfect for breakfast, as a dessert course, or, with the smaller blossoms, as an adornment to pudding or crème brûlée. As with beignets, it's best to serve right away while hot and crispy.
2 cups flour
2 tsp baking powder
2 tbsp corn starch
2 cups ice water
1. Check blossoms for insects. Usually they'll evacuate after their hiding place has been plucked.
2. Sift together flour, baking powder, and corn starch in a large bowl.
3. Stir in ice water.
4. Heat 1 inch of vegetable oil in a large saucepan on medium-high until a drop of water crackles and pops. Dredge blossoms in batter, allow excess to drip off, and carefully place in hot oil. Don't crowd the pan. Fry until lightly browned all over. Remove to paper towels.
5. Serve immediately while hot with a sprinkling of powdered sugar.
Note: I discovered on this occasion that I'm allergic to the pollen of big-leaf maples. After a few minutes of working in the kitchen with the blossoms, I was sniffling and my eyes were a red, teary mess. The upside is that this solves a recently developing mystery for me.