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Blackbourn was a hundred in the northwest of Suffolk, consisting of 66,272 acres (26,819 ha). Its northern county boundary is with Norfolk acros the River Little Ouse. The River Lark forms part of its western boundary. Most of its area was covered by the St. Edmundsbury District between 1974 and 2019, but on the east and south it stretches into the Mid Suffolk District.
The hundred appears in the Domesday Book of 1086 as Blachruna and Blackebrune meaning "black bourn", where a "bourn" is a stream. The stream in question is that which flows north through Ixworth, Bardwell, and Fakenham Magna to the Little Ouse at Euston and is still known as the Black Bourn.