I can only speak to dining in Blu on the S-Class ships. I did not see one person in jeans on our 16-night cruise...but then again, the ship had a European itinerary and I have found that many Euro itineraries, which hold more Brit and European guests, dress somewhat differently with a nod to tradition and custom.
On the M-class ship, yes more and more pax wear jeans on smart casual nights.
I agree with Don though about the humidity and heat and wearing jeans...ugh, just the thought makes me sweat,

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