I have a few black goldfish also, if you watch the video at about 2 minutes you will see one of the bigger black ones. It is about 6 to 8" There are a couple of smaller ones also but they may still change.
http://www.flickr.com/photos/robamyphotos/11951435133/in/set-72157634025922619/
I love your videos... I could watch that all day long. Just call me Mrs. Limpet.
And one of our black fish from last year is starting to show orange in that same pattern, from the bottom up. I like the black and orange - all black is rather stealth.