There are a number of comments on the answer to your meta, which dispute the way that flag was handled. Sometimes the community decides that us moderators are doing it wrong and they tell us to do it better next time. We're agents of the community's will, so unless we feel quite strongly, we typically shrug and go "by your command!" and then take whatever action is asked of us.
There's also a half dozen moderators, and each one of us works slightly differently. Some flags would be declined by one mod and acted on by another.
As far as black marks go, don't sweat flag disputes or declines. They have the tiniest meaning in the world. There's a concept called "flag weight" which determines where in the queue of moderator flags your flag will end up, and it has some unknown impact on that (now hidden) value when a flag gets declined. Otherwise, getting a declined flag means very little. The ordering in the queue is pretty pointless - typically when we look at flags we look at all of them, and there's typically good flags all the way down the page.
As long as you get the general concept of when to flag and/or for what reason, you'll be fine. And if I notice someone who is consistently flagging for the wrong reasons, I'll tend to decline with a custom reason that contains a link to a place to understand why.