I beg your pardon, but I made a mistake when I checked for the bug I described above: I didn't note that I was on a very slow internet connection.
It turned out that the blog pages I tested where not entirely loaded when I clicked on the thumbnail. After being fully loaded, the ColorBox pops up as expected on blog pages, no matter if »rel="box"« or »rel="box[g]"« is used.
So, everything is fine here – It was my fault.
Sorry for wasting your time and attention.
Thx & regards,