How we tested
We rounded up six nationally available blackberry jams, both seeded and seedless, and headed into the tasting lab to see which performed best in our cake and on its own. Not surprisingly, we preferred jams that actually tasted like blackberries and listed this fruit first on the label, indicating that it is the major ingredient. Several jams lost points for tasting of other fruits. Why would this be? Manufacturers may add concentrates and pectin from cheaper fruit sources (such as pears and grapes) to stretch pricier blackberries. We also liked sweeter jams. Finally, our tasters rejected jams that were pasty or gelatinous or had too many seeds. Our favorite won for its pleasing, complex flavor that made our tasters berry happy.