recent_blocks will return any blocks found in the last 24 or 72 hours (depending on the software version -- I changed it recently). If no blocks have been found in the last 72 hours, then the most recent block is not recent, and it doesn't show up there. Since the expected time per block is currently around 20 days on jtoomimnet and 80 days on mainnet, recent_blocks will usually show nothing at all.

https://blockchain.info/address/1Kz5QaUPDtKrj5SqW5tFkn7WZh8LmQaQi4 is a list of all Bitcoin p2pool blocks found. This includes both jtoomimnet and mainnet.