Had to google it:
Meanwhile, blueshirts represent the newest loophole for FBS programs to accumulate depth while not affecting their annual numbers. Like grayshirt recruits, blueshirts are counted toward the next season’s scholarship total.
There is one distinct difference, however. Unlike grayshirts, a blueshirt is able to enroll and participate in team events in the fall. This allows programs to get the best of both worlds: FBS schools can delay a scholarship for the following year, allowing them to over-sign beyond the 25-scholarship limit, and get the use of a prospect immediately rather than waiting until the following spring.
The use of a blueshirt was first popularized by New Mexico State, which would support its numbers by enrolling prospects right after the beginning of the fall semester and then place them on scholarship. Tennessee has also embraced the use of blueshirts under coach Butch Jones.