what a great pond you have! The waterfall is really cool.
The black stuff is just normal growth of ... I don't know exactly what, but it's normal. Probably a form of algae. I have the same exact stuff growing on all three outlets of my waterfall. When it builds up, I take a knife and scrape it off. What falls into the pond the fish eat. If I don't scrape it off, it eventually builds up and becomes long enough to reach from the falls to the water.
I don't think it has to do with time of year; I have it all the time. Of course, I'm in South Texas, where it's never really very cold.