Yes, it went under pressure - maximum torque as I set off from traffic lights. It's a Shimano STX. To be fair, it's fifteen years old, and has worked hard in that time.
Yes, I'm OK. And yes, that's why I'm buying cranks off you
The dark patch is definitely a propagated crack, and the lighter bit fresh metal. I should really have noticed that crack, but it was a bit round the back by the chainring, so not easy to see.
The bike has fallen over a time or two (I've been knocked off it at least once), so that could be a factor, but nothing very unusual for a touring bike.
MV, i think your guesses about the stress riser are correct.
