In my experience, all teas will grow darker over time--whether left on a heat source or not--and their tastes can become bitter.
I believe it's just continued exposure to air and oxidation, but it's not something I've tested.
The last time I brewed enough white to have a significant amount of left overs, I left a pitcher out on the counter. An hour later I thought, "Gee, does that look darker?" I left for work, and when I returned four hours later I thought, "Bloody hell, I'm sure I didn't brew a black!"
Same thing happens to greens, oolongs, and blacks. It's just not quite as noticeable or extensive in the latter two. I think if you remove the tea from the heat source and 'store' it in an airtight container, it shouldn't get quite as dark...but then again, it's not something I've tested personally.