Aqueon will make a tank with black silicone no problem (they use black silicone on all their bowfront tanks, just not their rectangular tanks). but there is a small extra charge.
but your LFS has to contact Aqueon and tell them directly that you want a tank specially made with black silicone. get them to ship it to the LFS and away you go. thats what I did.
here is what my 150g looked like empty (I am at home now so I can post pics)