Put lemon juice, almond milk, collagen, stevia, salt, and vanilla in blender. Turn on low for just a few seconds to mix. While blender is on low, slowly add in glucomannan. Blend on low for 30 seconds and turn off. Add in blackberries and ice cubes, blend on high until completely blended.
Depending on the sweetness of your blackberries, you may need to add a bit more sweetener.