Pick over berries carefully. Wash in cool water. Just use water that is left on berries after you wash them. Do not add more water. You should have only about 1 cup water to 4 cups of fruit or 6 quarts blackberries and 1 pint water).
Cook over medium heat until berries are softened and fully cooked (about 15 minutes). Crush berries thoroughly, place in a jelly bag and squeeze out juice. Let drain through jelly bag into a bowl for about 1 hour, then press the remaining juice through.
Measure juice and sugar. For each 2 pints of cooked blackberries, weigh 1 1/2 lbs. sugar.
In a wide, boiling pot, bring mixture slowly to a boil and heat to 222°F on a jelly thermometer. Skim off and remove any foam that rises to the surface. Stir continually or it will boil over. Add sugar, stirring until sugar has dissolved.
Bring to a hard boil again, stirring continually for 2 more minutes (it helps to heat up the sugar so that the temperature of the jelly will not be brought down too much when the sugar is added).
Be sure sugar is thoroughly dissolved. Pour into sterilized jars and seal.
Most traditional recipes do not call for further processing, but current government recommendations call for jelly to be processed in a boiling water bath canner for 10 minutes. Alternatively, you may process in a boiling water bath at 190°F for thirty minutes to pasteurize the jelly which retains more of the fresh taste, or the jelly may be frozen without processing.