Crust
1 (5 ounce) package graham crackers
    (about 10 or 11 crackers)
1/2 cup granulated sugar
2 tablespoons melted butter

Preheat the oven to 275 degrees F, with one shelf in the middle of the oven and one shelf lower.

Crush the graham crackers in a bowl, mix the sugar into the crumbs, then the melted butter. The mixture will still be crumbly. Press lightly into a greased 8-inch square baking dish.

Filling
8 ounces cream cheese
3/4 cup granulated sugar
1 egg
1 teaspoon vanilla extract
1 1/2 tablespoons lemon juice
1 cup sour cream
3 cups ripe blackberries

Beat the cream cheese with the sugar until smooth. Add the egg, vanilla extract and lemon juice. Mix until uniform, add the sour cream and beat for 1 minute.

Pour the cream cheese mixture over the crumb layer. Put a baking tray or similar container full of hot water on the lower shelf of the oven. Bake the cheesecake on the middle shelf for approximately 50 minutes. For the creamiest possible cheese layer, allow to cool slowly in the oven with the oven door ajar, up to an hour.

Rinse the blackberries and gently place them on paper towels to dry. When cheesecake is cool, arrange the berries on top.

Glaze
1 cup raspberry or blackberry jelly
1 1/2 tablespoons cornstarch
2 tablespoons cold water
2 tablespoons lemon juice

Melt the jelly in a small saucepan. Dissolve the cornstarch in 2 tablespoons cold water. Add the lemon juice and bring to the boiling point. While stirring the mixture, add the dissolved cornstarch. Cook 30 seconds or until thickened; remove from the heat.

Check the sweetness; depending on the sugar content of the jelly, you may wish to adjust by adding either lemon juice or sugar. Cool the glaze slightly. It should coat the back of a spoon. With a spoon, gently drizzle the glaze over the blackberries. Refrigerate, then cut into bars.