They may be rare, but they're more correct than black field boots!
Field boots were a kluge-- invented to cope with the difficulty of ankles and insteps in the pre-zipper era. Field boots were a compromise if you were built wrong and/or couldn't get customs. They were not worn in night classes.
All this means that if you have dress boots that look nice, you are ahead of the game.
I wear dress boots in hunters. I got custom boots for fox hunting and was assured that they are fine for the ring - and was darned if I was going to buy field boots just to be like (almost) everyone else.
Our trainer expects us to wear dress boots (with swagger tabs) in the show ring. It used to be required that anyone who went to the A shows with our barn had custom dress boots; this has been relaxed & some lucky folks who fit off-the-rack can wear pre-made (even field boots), but when we're measured for customs, everyone goes for dress boots - it is the most "correct", though definitely not the most "common" look