I had picked a small bowl full of blackberries and wanted to make jam with them. This only makes 1 pint and uses 3 cups of berries. Remember jam is softer than jelly. From Linda Ziedrich's The Joy of Jams, Jellies and other Sweet Preserves.
In a non-reactive pan, mash the blackberries with a potato masher. Add the sugar and heat the contents over medium heat, stirring gently, until the sugar is dissolved. Raise the heat to medium-high and boil the mixture until a drop mounds in a chilled dish (this took around 10 minutes for me).
Pour the jam into a jar and cap the jar tightly. Store in refrigerator. This jam should keep for several weeks.