It's a funny one, all this 'derby' malarkey. The media are very keen to use six broader 'regions' of England these days in relation to football, but very few would consider these regions if they were to put a tag on the area they're from. The six regions are NW, NW, WM, EM, SW and SE. Of course, that means the Blades are an NE club.
I'm from Kidderminster and I know of plenty of people who felt aggrieved when Sky (typically, but plenty of others besides) called the Birmingham/Black Country clubs v Stoke a 'West Midlands derby'. Of course, for Stoke, you could say that was the closest to a derby game they got in the PL, but nobody would refer to it as that as the 'West Midlands' term is used far more often to refer far more to the county than the wider region. Similarly, Blades v Newcastle next season could be referred to as a 'North East derby' as they are two sides from the NE region in the PL. Very few fans would think of it that way, as the 'North East' is typically used to refer to a very small area of the NE region, around Tyneside and Wearside.