"There are quite a lot of others sites also doing something to protest."

I don't think Reddit and about a dozen sites I never heard of count as — a lot of other sites"

"Should we, as PerlMonks, join in the effort?"

I may possibly be in the minority but I would say — NO

"What do you think?"

I doubt this would make a dent in the awareness campaign and I also don't see how it will do any good to inform visitors. While there are some parts of SOPA I don't agree with I support the overall goals of both SOPA and PIPA. I have spent a considerable amount of my time reading up on the issue (including reading the proposed bills) to come to this position. Yet I would NOT consider myself an expert by any definition. A black banner isn't going to 1. teach me something new. or 2. Change my mind.