Do you think adding Cocao powder to the flour ( in my case i going to bak chocolate rolls) has any effect to gluten development ?
I sometimes mix a bit of cocoa powder or coffee granules to my dough to add some flavour. I'm not sure if it's to do with development of gluten or not, but both of them certainly make dough a bit dryer and tighter, resulting with slightly denser crumb. So I usually add a bit more water and tiny amount of extra yeast to compensate for the effect.
BTW combination of coffee flavoured crumb with walnut pieces is great. I sometimes add a little amount of skimmed milk, too, when I'm after a slightly more rounded flavour. One of our family's favourite sourdough loaf with a twist. :)
I'm just guessing here, but I bet it depends on the kind of cocoa you use. Some cocoa powder is high in fat, which would inhibit gluten development. Also, some cocoa is "dutch process" meaning the natural acids have been neutralized. If it's not dutch cocoa, the acids remain. Certain acids will "strengthen" gluten.
...to darken the dough or for taste. I've seen no problems doing that. I've even substituted Nutella (for the sugar and oil) in a walnut bread once and loved the effect.