As in most cases it depends on the details.
The length of a blade (> 44mm and width > 10) is important and the circumstances.
Assume a fine to something up to € 10000, but a sentence up to 3 years is also possible.
In Germany it is not crime (anything that can be fined or minimal sentence is less than 1 year is not a crime).
I assume that they took your home address and let you leave the country, which alone is a sign that it not a crime.
Since you were so smart to place this in your luggage, where it garantied to be found by airport security, you will probably get something in the range of the minimal fine.
You also cooperated by giving a statement, which will assist to lessen the fine (you were after all caught red handed).
Having it on your person in a public space, the fine would have be higher.
Swinging it around among peaple in a threatening manner, will lead you into the range of a sentence.
When the fine arrives, there will be bank transaction form.
Go to the next post office or bank and pay it, retaining the receipt.
Border control will not be interested in you (they only get alerts for proper criminals).
Customs (Zoll) could be interested to enforce the fine, show them the receipt.
Pay the fine and get on with your life.
As to the U.S. Consulate (forget Embassy, State Department: they deal only in diplomatic affairs) all they will tell you is
- you must obey the laws of the country you are in
- assisted in getting an English speaking lawyer when requested
- send you bill for their efforts
A Consulate deals with citizen affairs (administration, assistance).
Most Embassies have a Cousulate department inside, but not all.
In Berlin it does not. The Cousulate is about 20 Kilometres away from the Embassy.
The right to call them is based on the Vienna Conventions 1815 and 1961. All countries that reconised these conventions are required to allow a foreigner to contact their Cousulate.
A dual citizen, when inside the country where they are a citizen, do not have this right.