I’m using use_fixed_fit_projection which is just fine for what I’m after, the black bars that it draws when the aspect ratio doesn’t align with the display - are those always black ?
I believe this is what you’re looking for:
Oh ok cool, but actually I want black 
I just wasn’t sure it might be different as I didn’t specify (unless it’s in a default setting someplace)
The default clear color can also be specified from game.project: https://defold.com/manuals/project-settings/#render-1