Powell arrives on a season-long loan from League One side Peterborough United, while Black has penned terms at Aldiss Park after impressing as a trialist during pre-season.
It has been a turbulent few weeks for the Magpies, who parted company with manager Adam Gusterson and assistant Olly Willis less than a week ago.
Tom Parke has since taken the reins and has moved quickly to appoint Dion Frary as his new captain, with the 2022/23 season fast approaching.
Meanwhile, the club have also confirmed that Matt Castellan will depart due to moving abroad.
Keep up-to-date with our exclusive email newsletters.
Subscribe