from sbnation, blueshirts can't play;
Shirt type On scholarship? Can play? Can practice? Meaning Redshirt Yes No Yes Athlete sits from competition for a year, then gets another year to complete four seasons. Grayshirt No No No Athlete waits an extra semester to become a full-time student and part of the team. Blueshirt Yes No Yes Athlete is not formally recruited, but is put on scholarship at start of freshman practice. Greenshirt Yes Yes Yes Athlete enrolls in school a semester early.
@TSchekler had it correct above. They can absolutely play -- it's just that when they do they are no longer classified as a blueshift. Same is true of a redshirt -- if you play him he's no longer a redshirt.