Soak the premordanted fiber in water for at least 1 hour.
Chop the blackberry leaves and stems. Put the chopped leaves and stems in a dye pot, and cover with enough water to immerse the fabric. Let the leaves steep overnight.
Bring leaves and stems to a simmer, 180°F (82°C), and simmer for 30 minutes, or until the color disappears from the plant parts and the water becomes a yellow-green. Turn off the heat. If you are happy with the color, strain the leaves from the dye bath. For darker colors, let the leaves continue to steep overnight. (If you are using a solar oven, let the plant material steep in the sun for 4 to 6 hours, or overnight.)
Add the wetted fiber to the dye pot, and bring to a simmer. Let the fiber simmer for 15 to 20 minutes, or until the fiber turns a light green. If you wish a darker shade, let the fiber steep overnight.
Remove the fiber from the dye pot, wash it with pH-neutral soap, rinse thoroughly, and hang to dry.