Did you torch it till it glows red or just torch a bit only? There are bound to be a little impurities that get deposited during the manufacturing process so it may turn black if you just torch it a bit. once the silica glows red hot all the impurities should get vaporized.
You should always torch silica till it glows white/red like in this video. The 1st part is torching the core of a zippo wick which is purportedly to be made of Silica as well but that thing melts. Skip to 30secounds to see real silica in action.