Crape Myrtles bloom in the summer. Depending on the variety, June through September. They are good for summer color, shade and they come in many colors, sizes and shapes, but they do nothing for the wildlife. I would stay away from some varieties (the non-Native American names) because they are magnets for aphids and mildew. If you are ever in the market for one, I would suggest Fanicks Nursery since they carry the largest selection and you can print their online list to narrow down what kind you might want.
As far as building a keyhole under any tree, the rule of thumb is NEVER pile soil or anything else up against a tree truck as this is the fastest way to kill it. So as long as the keyhole is away from the trunk you'll get shade but not sure the plants will like that. Perhaps a shade cloth for the summer would be a better option. My experience with my old garden and raised beds under live oaks/next to climbing roses and trumpet vines is after some time those plants will have their roots grow into your beds. I'm going to start building my keyhole later this year and then will have more experience on this matter then just all I've learned from Dr. Deb.