4 large firm but ripe plums (preferably Santa Rosa plums), halved, pitted, cut into ⅓-inch- thick wedges
1½ cups fresh blackberries
6 tablespoons sugar
½ vanilla bean, split lengthwise
Combine fruits and sugar in medium bowl. Scrape in seeds from vanilla bean; add bean. Toss to coat. Let stand at room temperature until juices form, tossing occasionally, about 1 hour.
DO AHEAD Can be made 3 hours ahead. Cover and chill.