There were only six episodes of the show after this one before it ended, but none of them featured Blair with any kind of a scar that would have resulted from the injury that is the focus of this episode. Not enough time passed between this and the next episode for Blair to have had plastic surgery that would fix such a scar, nor would it have been able to be covered simply by makeup. Since the plan was to carry Blair over to a spin-off series, it's questionable how they would explain away such a major continuity error.
As Casey is getting ready to leave the house, you can see Nancy McKeon's shadow along the upstairs wall as she waits for her cue to enter.