Actually, yes and no. A few days of heat, but unusual cool weather this time of year for us. Cool=75 to 85 degrees. Hot=90 to 95 degrees. Only had a few days of hot temps. I thought our weather was better for them overall.
I think maybe pirl was maybe thinking of scorch. If you water while it's super hot and/or sun is super strong it can cook them and get black or crispy tips. But, not usually black spots-usually the tender buds, blooms or new leaves edges will turn black.
Is it possibly Black Spot? I know roses get it, do dahlias? Usually, when I start getting funky growth or weird marks on leaves, I will dig down and make sure rot isn't starting or there isn't a jerky little bug eating the tubers or stalks.
I did this with one recently. Not sure what happened but the tuber wasn't rotted-the stalk just got pulled away from the tuber and was not healthy. Just cut it, sprayed where the stalk had been with Lysol and replanted. Sometimes it's just better to chuck the old unhealthy one and start with new growth, while it's still fairly early in the season? Of course, I say that, but it could just be an unhealthy/diseased tuber. Do you know the origin of the tuber-did you have it last year, purchase it, get it from friend, etc.?