You are correct the seed is the tiny black ball that comes after the flowers bloom. In zone 5 I do not have seeds yet as my canna's just began blooming. Seeds take a very long time to grow at least my experience is that. I have volunteer cannas's that appear in my yard from the previous year after cutting the foliage down and digging up for winter. To accelerate the process you should nick the seed to allow it to start to grow faster.

My seed pods are green and have short spines all over and yes they form just under where the bloom was. The seeds themselves are black but you won't see them until the pods are ripe. Other varieties may have darker pods but the majority that I have seen are green.