When specimens die and dry out quickly they often become much more blue/violet then they were in life. See how the head is still a greenish color? The entire beetle probably looked just like that before it died (It's extremely rare in that part of the country to see a live sexguttata that is part one color and part another).

And you're right that sexguttata is rarely out in the fall. They were known to have a fall diapause where only an extremely small fraction of the population would ever emerge. Interestingly, in the past five years or so they seem to fairly regularly emerge in the fall in larger numbers, even though this never used to happen. - Dan