Fritto misto, mixed fried foods, is a typical Italian nibble. Be sure to salt the asparagus and blossoms as soon as they're out of the oil so that the crystals will stick to the food.
Place flour in a medium bowl. Gradually whisk in seltzer water until smooth. Cover and refrigerate for 1 hour.
Meanwhile, bring a large pot of water to a boil. Place a large bowl of ice water by the stove. Plunge asparagus into the boiling water and cook for 1 minute. Lift it out with tongs or a slotted spoon and transfer to the ice water. Drain the asparagus and pat dry.
Combine olive oil and canola oil in a large skillet; heat over medium heat to 350 degrees F. Whisk the batter, then, working in batches, dip the asparagus and squash blossoms in the batter. Carefully transfer to the hot oil and fry until crisp and golden, 1 to 2 minutes per side, adjusting the heat as necessary to maintain the temperature, Transfer to a baking sheet lined with paper towels. Sprinkle with salt.