Blackburnshire (also known as Blackburn Hundred) was a hundred, an ancient sub-division of the county of Lancashire, in northern England. Its chief town was Blackburn, in the northwest of the hundred. It covered an area similar to modern East Lancashire, including the current districts of Ribble Valley excluding the part north of the wikipedia:River Ribble and east of the Hodder, which was then in Yorkshire), Pendle (excluding West Craven, also in Yorkshire), Burnley, Rossendale, Hyndburn, Blackburn with Darwen, and South Ribble (east from Walton-le-Dale and Lostock Hall).
Much of the area is hilly, bordering on the Pennines, with Pendle Hill in the midst of it, and was historically sparsely populated. It included several important royal forests. But in the 18th century several towns in the area became industrialized and densely populated, including Blackburn itself, and Burnley.
Post medieval period
The hundred of Blackburnshire continued to have administrative relevance until the abandonment of the system of hundreds in the early 19th century, long after feudalism ceased in England. As in other parts of England, the hundred was divided into parishes which were ecclesiastical parishes as well as being used for administrative purposes, like modern English civil parishes.
Early Blackburnshire had only two parishes, each covering a considerable but sparsely populated area. Whalley (now a much smaller parish) was the larger of the two, and covered the east of the hundred, while Blackburn parish covered the west. As mentioned above, the hundred was expanded to cover parts of the parishes of Chipping, Ribchester and Mitton. As Blackburnshire became more heavily populated and economically developed, the parishes of Whalley and Blackburn were split into many smaller parts over the centuries.
For more information, see the EN Wikipedia article Blackburnshire.