A blackberry is, apparently, actually many small berries in a cluster. I am not surprised, but I did not actually know that already.
1. A berry which grows in clusters, as grapes, currants, etc.; sometimes applied to the whole cluster. Obs.
2. Bot. One of the small fleshy berries or drupes which make up such compound fruits as the blackberry; sometimes applied to the compound fruit which they compose.
3. The stones or seeds of grapes and berries.
4. Anat. A racemose gland; a blind end of a duct of a secreting gland, which is divided into several lobes.
5. Anat. (See quot.)
1847 YOUATT Horse xiii. 297 There are, scattered through the substance of the liver, numerous little granules, called acini, from their resemblance to the small stones of certain berries.