No, black cross is not a satanistic coven.
Black cross is a coven which deals with its topics, and they can not be changed, and that is not a satanistic coven at all:
"Black Cross is a coven for those who wish to take an alternate look at things. Here we teach something this site has been missing, we teach the usual but we also teach the dark arts and the teachings of the Left Hand Path. We are not a coven which welcomes role-playing nor disrespect. We also have a prerequisite that you are at least moderately experienced before deciding to learn from us. ".
I can not see by this message that there is anything about satanism at black cross.
And now to your question, there are no satanistic coven in this site, and covens can not be opened right now.
There are few satanist members in this site, and i am sure there are little groups here (not considered to be so, but still)...
You can check out sites, or you can make your own little group by using mail/facebook/etc.