its safe. Miracles Aquariums in the GTA has been making tanks only with black silicone for 15 years now. Personally, I won't buy a tank with clear anymore.
black looks so much better, and doesn't stain or show up the algae as well.
your local Big Al's will have tubes of All Glass Silicone for sale. it is nicer looking (blacker) than the Perfecto black silicone.