How do I darken a video smoothly halfway through?
First of all the transition that you are willing to do is according to my knowledge an abrupt transition and according to me dimming 50% of brightness in just 1 second will look less smooth and more immediate than the dimming/reduction of brightness in 5 seconds.

Preferably, I would like you to use Adobe Premiere Pro and/or Adobe After Effects, if you like such transitions, in which "keyframes" do a nice job. 

But, in iMovie, you don't have a good keyframe feature. So, what you can do here is answered by AppleMan1958 (quoting it in his own words below):-


  Mar 9, 2012, 5:07 AM AppleMan1958 in response to Penfold941
  
  iMovie does not contain keyframes like FCP, but you can simulate it quite a bit. Just drag your mouse from your first "keyframe" to the end "keyframe" within a clip so that you have selected a range of clips. Then right-click within the range and select "Split Clip". Now you can apply a Ken Burns zoom to this section. This works for many effects.
  
  What you cannot do is keyframe the location of a motion graphic to simulate movement, for example.


You may read the full thread here

So, you might have got your answer, if there remains any other confusion, you may ask in the comment section below. But, try this solution out before asking or assuming any confusion. Hopefully, you might get rid of your problem yourself.