I believe you. Not only from the descriptive narrative of your post, but that you had a witness to this event. Date, time, place. I guarantee you were not the only one who saw that odd sight on that day.
There have been black helicopter theories for some time. Some say they are linked to drug trafficking, some have been linked to abductions, others say that they are privately owned. Whatever they are, and whatever their purpose, they seem to fly low enough to been seen, have no markings, and land, circle and observe private citizens. There are no aviation identification markers, and when people have phone local authorities, nobody knows anything about them.
Myself, I find it suspicious that I've never come across an 'official' explanation from any source that should be aware of them, ie airports, local police. It is like they don't exist.
Here is a link to other posts on the subject:viewtopic.php?f=20&t=3225&p=54913&hilit=black+helicopter#p54913