I have identified a placement that is doing well so I have a separate campaign where I whitelist this placement. Does it make sense for me also to blacklist this placement in my explorer (initial) campaign?
I think the reasoning for this is so that I am not bidding against myself for this particular placement however this also means the performance of my explorer campaign drops. Should I not keep it in the explorer campaign so that I have more chance to grab wins for both my campaigns (whitelist and explorer)?
You should blacklist it in your explorer campaign. Explorer campaign is for detecting pockets of profit (only use your best performing banners there).