I tried my Frontier Pro today with tap water from a bottle and black water came out. It cleared up after a few ounces but I want to be sure the filter is safe to us.
Your filter is perfectly safe to use and normal. What you are seeing is harmless carbon dust that is generated during the production of your Frontier Filter. It is ok to drink, but if your prefer not to you can remove it by sucking it and spitting it out until the stream is clear just as you have done.