That looks like a lot of precipitate, is it possible you dipped a brush into it or there was some other source of contamination?
I've noticed a fine black precipitate ( similar color as yours, but not nearly so much ) forms over a long time in silver nitrate solution. It's always perfectly clear when I first make it but over a period of months and years the precipitate sticks to the bottle and glass eyedropper in it. Just a few weeks ago I switched to a new clean eyedropper bottle after using the old one for about 5 years ( it's been refilled many times ) because of this. My ( maybe wrong! ) idea is a very slow reaction with dissolved gasses, like maybe carbonic acid. The silver oxide I've seen is more brown, but maybe there is variation in its color. I do have a sense that it happens more when the bottle is mostly empty. Anyway, it's never caused me any problems and I don't know that it was really necessary for me to use a new eyedropper bottle.
My fixer bottles are more fun.. the silver precipitates out as a mirroring on the glass that is very pretty!