Been wrestling with this same issue for some time, and finally found a workaround.
Using Xcode 7.2.
Not sure exactly why this is happening, but Over Current Context is not an available option. This project was created some time ago, which I have a hunch could be related.

As a test, I created a brand new project, and Over Current Context is an option.

Since deleting and recreating my existing project was out of the question, I ended up doing the following:
- Set the - Presentationon the segue to- Default. This is preparation for the next step.
 
- On the destination view controller scene, set - Presentationto- Over Current Contextwithin the- Attributes Inspector.
 

I was able to successfully see the parent view controller underneath the semi-transparent modal. 
Note: 
I'm supporting iOS 8 and above, so I don't have to mess around with setting the modalPresentationStyle on the parent vs. the child.
Failed attempts:
- Setting - modalPresentationStyleto- .OverCurrentContexton the destination view controller in- prepareForSeguedid not work for me, because the Storyboard setting seemed to override my code.
 
- Setting - Defines Contextand- Provides Contextdid not seem to make a difference. I'm guessing these could come into play for some app architectures, just not my in my particular project.