In my observation my keyword analysis in statcounter for my blog is getting lower when there's always a comment to my blog...
i think comments on blog has nothing to do on keywords analysis.. Keyword analysis in statcounter tells you the percentage of certain keywords used/typer by users in any search engine to reach your site..
then you've got fewer people finding your blog through search engine searches - you've possibly got more repeat visitors that are making comments, rather than new visitors.
I am not aware of keyword analysis with statcounter, but does that analyze tell you the keyword density of the site? If yes, than it is more normal that the density decreases when you get more comments, if those commentators don't use your keywords in their comment. Nothing weird, "Lets Say Keyword" the density of the word keyword in that phrase alone would be around 33.33% but if a commentator came and made a comment like "Ok cool" the density of the keyword word would decrease to 25% which is pretty normal.