Juice lemons to get 1 cup juice. Combine juice with the first 4 ingredients in a high-powered blender. Blend until completely smooth, about 60 seconds.
Place a fine mesh sieve over a wide-mouth pitcher or wide bowl with a spout. Pour blended mixture through, pressing to extract all you can through. I like to chill this mixture for an hour if time permits.
Pour strained blackberry mixture into a pitcher, then gradually add cold water until lemonade is to your liking. I start with 4 cups and taste, then adjust from there. I like it best with about 5 cups depending on sweetness of berries. Keep in mind, ice will water it down as well. Pour over ice and enjoy!
Notes
Storage: Store this lemonade in the fridge (without ice) for up to 4 days. When you’re ready to serve, pour it over ice and enjoy!