What is this and where can I find it? I'm located in Toronto... downtown.
It's a traditional chinese ingredient, espec in szcheuan cooking. Very very dark in color, richer in flavor than standard clear rice vinegar. I got my large, inexpensive bottle at an asian grocery (far, far from toronto, so I can't help you with that part).
There are a few varieties; I'm most familiar with two of them. The ChinKiang/Zhengjiang black vinegars from the namesake city in Jiangsu are used extensively in the cooking around that province. (Jiangsu cooking is one of the pillars of Shanghainese cuisine.) Shanxi also has a dark vinegar of their own that's perhaps a bit more pungent; Northern Chinese seem to prefer it (well, at least my Northern Chinese friends).
These are the more commonly available ones at least in the US, and one might see them used in restaurants that specialize in Chinese cuisines from other regions.
I believe there's a version from Sichuan as well, which contributes to some of their traditional dishes like hot and sour soup.