Hi Redrose,
That is odd about the black bits? I don't have an answer for you on that one, but the dark color is probably a sign of infection or protein (which you know you have). I did a 24 hour urine a few months ago and turned out I couldn't hand it in on the weekend either. So I was told to keep it in the fridge. Gross, yes. But as long as it stays cold, it will still be a useful sample when you hand it in on Monday.
Hope this helps. I'd be interested to know what the black bits are.
Ginny