Black spot on hip?

I let Pierre Cardin set a few OP hips. They are about 80-85 days in.
About a week ago the ripest hip started to get a slightly darker area, which has now become a large black spot. Is this fungus?

The plant is healthy aside from some fried leaves from the extra-long heatwave we have been experiencing for the last three weeks (thankfully temps are coming down just a little now). There has been no rain at all, but air humidity is on the high side as always here.
I checked for critters but aside from the usual ants, nothing seems to be on this rose or her immediate neighbors.
This morning I cut the hip off in the hopes that whatever it is doesn’t spread to the other hips.
Opening it shows that whatever it was seems to have only affected the surface layer of the hip. The seeds inside were immature, but otherwise looked fine.

Now I am wondering if I should have just left it on? Is this benign or was the hip doomed anyway?

It looks like sun scald to me. It’s common. The hip tissue is pretty much dying as it turns color. Direct sun shining on it can literally cook it, burn it. It’s normal in my experience. It won’t “spread” as it is a function of the hot, direct sun as you mention. If you wish to prevent it, create something to provide some shade from the worst of the direct sun, while still allowing for air circulation to prevent heat build-up.

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Thank you so much @roseseek !
Sunscald makes sense… Darn, I could have left it on then! I’ll try to come up with something to shade the others. Thanks again for your advice!

You’re welcome! In all the literal decades I’ve harvested hips, the only times the blackened sun scald was an issue was when the entire hip was fried. A round spot like your photo demonstrated hasn’t appeared to cause any issues.