So, I've been quiet because I've been waiting for the correct gelatin to arrive. Tonight I repeated the process and the results are quite the same. The temperature, filtration and nitrate addition was followed as suggested, but no signs of any black; all I got is dark grey. The only possible fault I can think of is that the stainless steel thermometer affected the reaction, but I am not sure if that actually happened since it is a laboratory thermometer. I am really clueless now..