Answer by kbrinner for How to override aggregator_block_view() in the aggregator module?

You don't need to override the block that is automatically provided by Aggregator. Much easier to make your own block using Views.

1. Go to Views -> Add a New View
2. Show Aggregator Item
3. You can uncheck 'create a page' and check 'create a block' if you only want a block.
4. Give the block a name and Continue & edit
5. Under Fields, add the Aggregator Title and any other fields you want.

If you have multiple feeds, give them categories in the Aggregator settings (admin/config/services/aggregator), and then filter by category ID within the view.