Why Is My Mint Turning Black?

I checked on my garden the other day and to my surprise, some of my mint leaves were black. Like this:

Black mint

My first thought: It has some plant disease I've not heard of. But then I did as I always do in these situations, and I Googled it. I often feel like I garden by Google these days. I mean, how did gardeners get by without Google? I'm only partly kidding ...

At any rate, it appears that mint sometimes turns black when it has had too much water. It's been very hot, so I've been watering every day, when the soil looked dry. But I'm going to switch to the strategy of waiting until the mint looks like it needs water, and then water it, and see whether that takes care of this little problem.