Hm... It looks as if it works! I just checked the little spot and the ink dried up and stayed black. I then took my cloth with black ink/water mix and gently rubbed the area again to see what happened. And to my surprise it stayed black

Apparantly, the stampink is so concentrated that even the glue cannot stay unstained...

So I took the plunge and treated all the gluespots with a thin layer of unmixed ink, just right out of the bottle. I'll now let it dry for a couple of hours and then, fingers crossed, I can finish the edge with a black burst. One thing I know, the mix for the black burst must be concentrated much more then the mix I have now.