Former top amateur Billy Joe Saunders was taken the distance by Nick Blackwell before adding the British title to his Commonwealth middleweight strap.
Saunders was never in serious trouble but 22-year-old Blackwell made him look uncomfortable for the first time in a professional ring.
Saunders had the better of the early exchanges but Blackwell appeared unperturbed and gradually clawed his way back into it.
The challenger walked his opponent down despite taking Saunders' heavier shots, having plenty of successes of his own and opening up a cut by the champion's right eye.
Blackwell, whose only previous defeat was to Martin Murray, proved his fitness as he came on strongly past the halfway mark, backing Saunders up and making him look sluggish at times.
But Saunders' early lead and ultimately classier work made him a deserved winner in his third title defence of the year, the judges scoring it 117-112, 116-113 and 115-114.
Earlier in the night, Liam Smith won the vacant Commonwealth light-middleweight title with a unanimous points victory over game Londoner Steve O'Meara.
Smith becomes the third of four boxing brothers to claim a domestic title.
