Blackberries have seeds. If you don't like the texture of the seeds, you can strain the sauce to take them out. This changes the recipe some, since you would need to strain the sauce after the fruit breaks down but before it thickens, and then return it to the stove, bring it back to a boil, and add the corn starch. Straining will also remove some of the other matter and tend to make the sauce more like a syrup. My preference is to leave them in and, in fact, to have a lumpy sauce that has the texture of preserves rather than jelly or syrup.