inertia,
I've done that before as well, using an onclick event to set the div as visible. There's nothing "black hat" about it just as long as the "click here" link is visible and it's obvious that something is going to happen if you click the link. What really matters is your intent.
Were it me, I'd try to use some descriptive keywords that would provide users with a clue as to what's inside the div.
Besides, what you're describing might be better explained as an "expanding" layer, instead of "hidden" text.
Here's a link to an article in Matt Cutts' blog related to this very subject:
SEO Mistakes: Unwise comments