I had wanted to elaborate on that last sentence a bit more but ran out of space to finish it (another annoying product of Civil).  By "black mark" I meant by hanging around too long, or worse, being a detriment to the Senate being able to pass anything (which is already a difficult enough task with the current Senate make-up).  

If he did hang onto the seat but left it vacant for months (especially now, and at this age), I think that would unnecessarily leave a black mark on the end of what's a great career of service to the country.  I would hate for people to think of that as his final chapter as a Senator.