Welcome Norbert, we are honored to have your European perspective. You will learn a lot on this forum, I am amazed at the accumulative knowledge here, often freely shared.
Your question about what exactly is a black tail compared to a unicolor has been the source of many debates and not many definitive answers. We have also seen the intermediate colors, and even seen black tails fade into unicolors! Some of us think they may be the same species, just extreme morphs. Some think the distinction is in marketing, that the buyers want coal black black tails and honey gold unicolors, and the intermediates are somehow less desired. I certainly can't answer your question, I wonder if others will jump in and debate it again? You may want to search the archives on this topic.
It will be fun to hear from you, the Europeans have made many of the great gains in herpetoculture and we know some incredible D. couperi reside in Italy, the Netherlands and Belgium, and Germany. Welcome, vic h.