How blended is your ad with your content? Is there a border around it or anything to set it apart?
I put "Sponsored" above my ads and suffered no ill effect, but these are sidebar ads. If I had an ad in the content area and the CTR suffered that much by labeling it, then I'd be worried that too many people were clicking on it accidentally.
Of course it's up to you, but I don't think you need to label it. I don't label my ad blocks because I've never wanted to run afoul of Google's policies around how to present ads. Also, I think most users by now know that Google's ads are just that: ads.