Does the fact that I use Google adsense deprioritize me with Yahoo?
If that were to happen, the AdSense forums would be lit up with all of us connecting the conspiracy dots. But it's not happening so it's more than safe to say that Yahoo isn't penalizing adsense sites.
Could I be on a Yahoo blacklist for other reasons?
Being partially indexed is hardly a blacklist, so it's a stretch to use that kind of language for your situation.
If the content in question is Forum content, then my opinion (and feel free to disagree with it) is that there may not be enough unique content on the page to make it worth indexing.