Copper sulfate is used to destroy tree roots, so check at a hardware store. But for some reason, I found some at a pharmacy. No, HP alone won't do anything that I'm aware of.
Now remember, this is for an unexposed/unprocessed plate, but expose the hell out of it (i.e. take it outside and look at it for a long time in the bright sunshine... take your time). Then, develop it in D76 above 20° for any time greater than, let's say 10 minutes. This should definitely result in a black plate.
Technically the copper-sulfate/hydrogen-peroxide mixture is a bleach-etch. Here's a thread about it... http://www.apug.org/forums/forum37/1...al-bleach.html
If you are serious, we can look harder for a formula. I'm just a little lazy at the moment...