Combine blueberries, strawberries, onion, lime zest, juice and cilantro in a food processor or blender and pulse. The consistency is up to you but I like to leave mine a bit chunky, so I only pulse it about 5-6 times. Taste and season with salt and pepper if desired. Scrape salsa into a bowl and fold in chopped avocado. Serve with pita or corn chips, or on top of fish or chicken.