Do you know how many Senators in blue States are up of election in 2016? I don't.
I thought I'd heard it was something in the mid-20s. A quick search comes up with 34 total seats - 24 currently held by Republicans & 10 by Democrats. If the Dems hold serve and pick off only 3 +/- of those 24 Republicans, control switches back. (Depends on how AK, LA, & VA shake out, too.)
Of course, that doesn't include any special elections that might pop up in the interim, or Senators changing parties.
Not saying it's a guaranteed loss for the GOP, but it's certainly not a slam dunk that the leadership (Senate Majority & Committee heads) doesn't revert back to the Socialist .. I mean .. Democrat Party.