I really think if they could have come up with some other term than Sector Block it would be more clear. Quadrant - perhaps, but then the purists would be screaming about Alpha/Beta/Gamma.
Hmmm... a parsec is a distance unit, worth about 3.26 Light Years or 19 trillions of miles. The term really wouldn't make sense as an area designation.
To the OP: As far as I know, there is no way to search for a system outside of your current sector block. As previous posters have said, if the sector block or sector are not in your mission briefing, bug report it and it will be fixed. good luck!