Since you served in the Navy- I'll ask you
What flag waved over them Slave ships? USA of Confederate? Did they dock in a Northern port?
You're talking about a long period of time. Slavery ran from the early 1600's to the mid-1800's. During most of that period, slave ships flew the flags of European countries. After the U.S. gained its independence then a lot of U.S. ships participated in the slave trade, even after it was outlawed in 1809. And for a brief period between 1861 and 1865 the confederate flag flew over ships that ran a small number of slaves into the confederacy.
Now as to whether they landed in Northern ports, I'm sure that they did when slavery was legal and slave importing was legal. Just as they also landed in Southern ones.
Happy?