Since you're on an outgoing flight, you won't go through customs in the U.S. It's still possible, but not likely that your luggage will be flagged.
I hope you have a TSA approved lock on your suitcase. That way if there's a concern, TSA will open your suitcase and check. If not, buy one before you leave. The TSA has the authority to break a lock if they have reason to inspect the contents of a suitcase.
If your suitcase is opened you'll find a note inside from TSA.
Bring the Blueberry jam to your Japanese family and don't worry about it.
All of these procedures, though sometimes overdone and frustrating...are meant to keep us safe.
I envy you your trip as Japan is high on my bucket list!