Yes, grandmom did use shortening when she baked her fresh blueberry cake, so I do too. This cake is just too amazing for me to be willing to alter it in any way. But you certainly can substitute an equal amount of butter or margarine for the shortening.
I researched how to do this with the best results: Always start with butter at room temperature. And (I thought this was interesting) the bowl and beaters should be cool to prevent the butter from becoming too warm as you beat it. To accomplish this easily, you could rinse both the bowl and the beaters in very cold water. Then make sure you cream the butter and sugar for 4 to 5 minutes until the mixture is truly light and fluffy, occasionally scraping down the sides of the bowl with a spatula.