Well, the image you uploaded shows no black bar inside the actual frame (the grey bounding box).
That means that what causes the black bar, is that your project's width and height don't match with the browser rendering window.
Could you try exporting to NW.js (all three checkboxes unchecked) and tell me what you see?
On nw.js it shows perfect. I think this has to do with the server configuration maybe. It adds that black line for some weird reason.
When I open the project on my browser from construct 2 run, it works fine.
When I upload the game, it adds the black bar.
The game has a resolution of 960x640.