In Jainism, the consumption of fruits is generally allowed, including jamun (Indian blackberry), as long as certain guidelines are followed:

1. Ripeness: The fruit must be fully ripe and not infested with insects or worms.
2. Non-violence (Ahimsa): Care should be taken that consuming the fruit does not involve violence to living beings. Fruits with many seeds or those that may have tiny life forms inside are sometimes avoided by stricter followers.
3. Seasonality: Jains typically prefer to consume seasonal fruits, and jamun is a summer fruit in India.
4. Digambar and Shwetambar Practices: Some stricter sects or monks may have additional restrictions, but for laypersons (Shravaks and Shravikas), jamun is generally considered permissible.

However, always consult your local Jain guru or community leader for guidance, as practices may vary.