Why did my last trip give me a bum blister?
I worked it out - its the cleats and riding a MTB trail that caused my blister.

Hows that?  I noticed today that when there's a mound or little crest, I've been locking up my legs and the rear wheel gets lifted up by the cleats.  When I'm off the saddle, sitting back down drives the chamois pad downwards.

The fix was to wear a belt with my bike pants to hold them up, or to take them in a bit next time the sewing machine is set up.   Or buy a smaller size.  Or wear normal pants over the top of the padded biking pants.