Have look at the answer below, which may answer most of your questions.
The important point is:
Did you, as a adult, ask for and recieved a foreign nationality?
when no: apply for Staatsangehörigkeitsausweis
This assumes you were German through birth (before foreign birth registration)
one of your parents were German
germany - Does ...
Since 28. August 2007, where in Section 25 (1) sentence 2 StAG was added.
§ 25 Abs. 1 Satz 2 StAG
Der Verlust nach Satz 1 tritt nicht ein, wenn ein Deutscher die Staatsangehörigkeit eines anderen Mitgliedstaates der Europäischen Union, der Schweiz oder eines Staates erwirbt, mit dem die Bundesrepublik Deutschland einen völkerrechtlichen Vertrag nach § ...
The current state of the relevant law may be found at 8 USC 1321(a). This contains another condition that you do not mention, which is that the child must have a green card:
(a) In general A child born outside of the United States automatically becomes a citizen of the United States when all of the following conditions have been fulfilled:
(1) At ...
In such cases, it may also depend on the type of residence permit you had before the issue date of the Blue Card. In similar situation, I got an answer from Auslaenderbehoerde that the 21 month can only be counted from the issue date of the Blue Card. From my understanding of the law, I consider this as the most correct answer. On the other hand, I know an ...